{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "#hide\n",
    "! [ -e /content ] && pip install -Uqq fastbook\n",
    "import fastbook\n",
    "fastbook.setup_book()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "#hide\n",
    "from fastbook import *"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# A fastai Learner from Scratch"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Data"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "path = untar_data(URLs.IMAGENETTE_160)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "t = get_image_files(path)\n",
    "t[0]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "from glob import glob\n",
    "files = L(glob(f'{path}/**/*.JPEG', recursive=True)).map(Path)\n",
    "files[0]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "im = Image.open(files[0])\n",
    "im"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "im_t = tensor(im)\n",
    "im_t.shape"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lbls = files.map(Self.parent.name()).unique(); lbls"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "v2i = lbls.val2idx(); v2i"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Dataset"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Dataset:\n",
    "    def __init__(self, fns): self.fns=fns\n",
    "    def __len__(self): return len(self.fns)\n",
    "    def __getitem__(self, i):\n",
    "        im = Image.open(self.fns[i]).resize((64,64)).convert('RGB')\n",
    "        y = v2i[self.fns[i].parent.name]\n",
    "        return tensor(im).float()/255, tensor(y)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "train_filt = L(o.parent.parent.name=='train' for o in files)\n",
    "train,valid = files[train_filt],files[~train_filt]\n",
    "len(train),len(valid)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "train_ds,valid_ds = Dataset(train),Dataset(valid)\n",
    "x,y = train_ds[0]\n",
    "x.shape,y"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "show_image(x, title=lbls[y]);"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def collate(idxs, ds): \n",
    "    xb,yb = zip(*[ds[i] for i in idxs])\n",
    "    return torch.stack(xb),torch.stack(yb)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "x,y = collate([1,2], train_ds)\n",
    "x.shape,y"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class DataLoader:\n",
    "    def __init__(self, ds, bs=128, shuffle=False, n_workers=1):\n",
    "        self.ds,self.bs,self.shuffle,self.n_workers = ds,bs,shuffle,n_workers\n",
    "\n",
    "    def __len__(self): return (len(self.ds)-1)//self.bs+1\n",
    "\n",
    "    def __iter__(self):\n",
    "        idxs = L.range(self.ds)\n",
    "        if self.shuffle: idxs = idxs.shuffle()\n",
    "        chunks = [idxs[n:n+self.bs] for n in range(0, len(self.ds), self.bs)]\n",
    "        with ProcessPoolExecutor(self.n_workers) as ex:\n",
    "            yield from ex.map(collate, chunks, ds=self.ds)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "n_workers = min(16, defaults.cpus)\n",
    "train_dl = DataLoader(train_ds, bs=128, shuffle=True, n_workers=n_workers)\n",
    "valid_dl = DataLoader(valid_ds, bs=256, shuffle=False, n_workers=n_workers)\n",
    "xb,yb = first(train_dl)\n",
    "xb.shape,yb.shape,len(train_dl)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "stats = [xb.mean((0,1,2)),xb.std((0,1,2))]\n",
    "stats"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Normalize:\n",
    "    def __init__(self, stats): self.stats=stats\n",
    "    def __call__(self, x):\n",
    "        if x.device != self.stats[0].device:\n",
    "            self.stats = to_device(self.stats, x.device)\n",
    "        return (x-self.stats[0])/self.stats[1]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "norm = Normalize(stats)\n",
    "def tfm_x(x): return norm(x).permute((0,3,1,2))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "t = tfm_x(x)\n",
    "t.mean((0,2,3)),t.std((0,2,3))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Module and Parameter"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Parameter(Tensor):\n",
    "    def __new__(self, x): return Tensor._make_subclass(Parameter, x, True)\n",
    "    def __init__(self, *args, **kwargs): self.requires_grad_()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "Parameter(tensor(3.))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Module:\n",
    "    def __init__(self):\n",
    "        self.hook,self.params,self.children,self._training = None,[],[],False\n",
    "        \n",
    "    def register_parameters(self, *ps): self.params += ps\n",
    "    def register_modules   (self, *ms): self.children += ms\n",
    "        \n",
    "    @property\n",
    "    def training(self): return self._training\n",
    "    @training.setter\n",
    "    def training(self,v):\n",
    "        self._training = v\n",
    "        for m in self.children: m.training=v\n",
    "            \n",
    "    def parameters(self):\n",
    "        return self.params + sum([m.parameters() for m in self.children], [])\n",
    "\n",
    "    def __setattr__(self,k,v):\n",
    "        super().__setattr__(k,v)\n",
    "        if isinstance(v,Parameter): self.register_parameters(v)\n",
    "        if isinstance(v,Module):    self.register_modules(v)\n",
    "        \n",
    "    def __call__(self, *args, **kwargs):\n",
    "        res = self.forward(*args, **kwargs)\n",
    "        if self.hook is not None: self.hook(res, args)\n",
    "        return res\n",
    "    \n",
    "    def cuda(self):\n",
    "        for p in self.parameters(): p.data = p.data.cuda()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class ConvLayer(Module):\n",
    "    def __init__(self, ni, nf, stride=1, bias=True, act=True):\n",
    "        super().__init__()\n",
    "        self.w = Parameter(torch.zeros(nf,ni,3,3))\n",
    "        self.b = Parameter(torch.zeros(nf)) if bias else None\n",
    "        self.act,self.stride = act,stride\n",
    "        init = nn.init.kaiming_normal_ if act else nn.init.xavier_normal_\n",
    "        init(self.w)\n",
    "    \n",
    "    def forward(self, x):\n",
    "        x = F.conv2d(x, self.w, self.b, stride=self.stride, padding=1)\n",
    "        if self.act: x = F.relu(x)\n",
    "        return x"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "l = ConvLayer(3, 4)\n",
    "len(l.parameters())"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "xbt = tfm_x(xb)\n",
    "r = l(xbt)\n",
    "r.shape"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Linear(Module):\n",
    "    def __init__(self, ni, nf):\n",
    "        super().__init__()\n",
    "        self.w = Parameter(torch.zeros(nf,ni))\n",
    "        self.b = Parameter(torch.zeros(nf))\n",
    "        nn.init.xavier_normal_(self.w)\n",
    "    \n",
    "    def forward(self, x): return x@self.w.t() + self.b"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "l = Linear(4,2)\n",
    "r = l(torch.ones(3,4))\n",
    "r.shape"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class T(Module):\n",
    "    def __init__(self):\n",
    "        super().__init__()\n",
    "        self.c,self.l = ConvLayer(3,4),Linear(4,2)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "t = T()\n",
    "len(t.parameters())"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "t.cuda()\n",
    "t.l.w.device"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Simple CNN"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Sequential(Module):\n",
    "    def __init__(self, *layers):\n",
    "        super().__init__()\n",
    "        self.layers = layers\n",
    "        self.register_modules(*layers)\n",
    "\n",
    "    def forward(self, x):\n",
    "        for l in self.layers: x = l(x)\n",
    "        return x"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class AdaptivePool(Module):\n",
    "    def forward(self, x): return x.mean((2,3))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def simple_cnn():\n",
    "    return Sequential(\n",
    "        ConvLayer(3 ,16 ,stride=2), #32\n",
    "        ConvLayer(16,32 ,stride=2), #16\n",
    "        ConvLayer(32,64 ,stride=2), # 8\n",
    "        ConvLayer(64,128,stride=2), # 4\n",
    "        AdaptivePool(),\n",
    "        Linear(128, 10)\n",
    "    )"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "m = simple_cnn()\n",
    "len(m.parameters())"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def print_stats(outp, inp): print (outp.mean().item(),outp.std().item())\n",
    "for i in range(4): m.layers[i].hook = print_stats\n",
    "\n",
    "r = m(xbt)\n",
    "r.shape"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Loss"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def nll(input, target): return -input[range(target.shape[0]), target].mean()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def log_softmax(x): return (x.exp()/(x.exp().sum(-1,keepdim=True))).log()\n",
    "\n",
    "sm = log_softmax(r); sm[0][0]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "loss = nll(sm, yb)\n",
    "loss"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def log_softmax(x): return x - x.exp().sum(-1,keepdim=True).log()\n",
    "sm = log_softmax(r); sm[0][0]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "x = torch.rand(5)\n",
    "a = x.max()\n",
    "x.exp().sum().log() == a + (x-a).exp().sum().log()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def logsumexp(x):\n",
    "    m = x.max(-1)[0]\n",
    "    return m + (x-m[:,None]).exp().sum(-1).log()\n",
    "\n",
    "logsumexp(r)[0]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def log_softmax(x): return x - x.logsumexp(-1,keepdim=True)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "sm = log_softmax(r); sm[0][0]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def cross_entropy(preds, yb): return nll(log_softmax(preds), yb).mean()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Learner"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class SGD:\n",
    "    def __init__(self, params, lr, wd=0.): store_attr()\n",
    "    def step(self):\n",
    "        for p in self.params:\n",
    "            p.data -= (p.grad.data + p.data*self.wd) * self.lr\n",
    "            p.grad.data.zero_()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class DataLoaders:\n",
    "    def __init__(self, *dls): self.train,self.valid = dls\n",
    "\n",
    "dls = DataLoaders(train_dl,valid_dl)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Learner:\n",
    "    def __init__(self, model, dls, loss_func, lr, cbs, opt_func=SGD):\n",
    "        store_attr()\n",
    "        for cb in cbs: cb.learner = self\n",
    "\n",
    "    def one_batch(self):\n",
    "        self('before_batch')\n",
    "        xb,yb = self.batch\n",
    "        self.preds = self.model(xb)\n",
    "        self.loss = self.loss_func(self.preds, yb)\n",
    "        if self.model.training:\n",
    "            self.loss.backward()\n",
    "            self.opt.step()\n",
    "        self('after_batch')\n",
    "\n",
    "    def one_epoch(self, train):\n",
    "        self.model.training = train\n",
    "        self('before_epoch')\n",
    "        dl = self.dls.train if train else self.dls.valid\n",
    "        for self.num,self.batch in enumerate(progress_bar(dl, leave=False)):\n",
    "            self.one_batch()\n",
    "        self('after_epoch')\n",
    "    \n",
    "    def fit(self, n_epochs):\n",
    "        self('before_fit')\n",
    "        self.opt = self.opt_func(self.model.parameters(), self.lr)\n",
    "        self.n_epochs = n_epochs\n",
    "        try:\n",
    "            for self.epoch in range(n_epochs):\n",
    "                self.one_epoch(True)\n",
    "                self.one_epoch(False)\n",
    "        except CancelFitException: pass\n",
    "        self('after_fit')\n",
    "        \n",
    "    def __call__(self,name):\n",
    "        for cb in self.cbs: getattr(cb,name,noop)()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Callbacks"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Callback(GetAttr): _default='learner'"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class SetupLearnerCB(Callback):\n",
    "    def before_batch(self):\n",
    "        xb,yb = to_device(self.batch)\n",
    "        self.learner.batch = tfm_x(xb),yb\n",
    "\n",
    "    def before_fit(self): self.model.cuda()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class TrackResults(Callback):\n",
    "    def before_epoch(self): self.accs,self.losses,self.ns = [],[],[]\n",
    "        \n",
    "    def after_epoch(self):\n",
    "        n = sum(self.ns)\n",
    "        print(self.epoch, self.model.training,\n",
    "              sum(self.losses).item()/n, sum(self.accs).item()/n)\n",
    "        \n",
    "    def after_batch(self):\n",
    "        xb,yb = self.batch\n",
    "        acc = (self.preds.argmax(dim=1)==yb).float().sum()\n",
    "        self.accs.append(acc)\n",
    "        n = len(xb)\n",
    "        self.losses.append(self.loss*n)\n",
    "        self.ns.append(n)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "cbs = [SetupLearnerCB(),TrackResults()]\n",
    "learn = Learner(simple_cnn(), dls, cross_entropy, lr=0.1, cbs=cbs)\n",
    "learn.fit(1)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Scheduling the Learning Rate"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class LRFinder(Callback):\n",
    "    def before_fit(self):\n",
    "        self.losses,self.lrs = [],[]\n",
    "        self.learner.lr = 1e-6\n",
    "        \n",
    "    def before_batch(self):\n",
    "        if not self.model.training: return\n",
    "        self.opt.lr *= 1.2\n",
    "\n",
    "    def after_batch(self):\n",
    "        if not self.model.training: return\n",
    "        if self.opt.lr>10 or torch.isnan(self.loss): raise CancelFitException\n",
    "        self.losses.append(self.loss.item())\n",
    "        self.lrs.append(self.opt.lr)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lrfind = LRFinder()\n",
    "learn = Learner(simple_cnn(), dls, cross_entropy, lr=0.1, cbs=cbs+[lrfind])\n",
    "learn.fit(2)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "plt.plot(lrfind.lrs[:-2],lrfind.losses[:-2])\n",
    "plt.xscale('log')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "class OneCycle(Callback):\n",
    "    def __init__(self, base_lr): self.base_lr = base_lr\n",
    "    def before_fit(self): self.lrs = []\n",
    "\n",
    "    def before_batch(self):\n",
    "        if not self.model.training: return\n",
    "        n = len(self.dls.train)\n",
    "        bn = self.epoch*n + self.num\n",
    "        mn = self.n_epochs*n\n",
    "        pct = bn/mn\n",
    "        pct_start,div_start = 0.25,10\n",
    "        if pct<pct_start:\n",
    "            pct /= pct_start\n",
    "            lr = (1-pct)*self.base_lr/div_start + pct*self.base_lr\n",
    "        else:\n",
    "            pct = (pct-pct_start)/(1-pct_start)\n",
    "            lr = (1-pct)*self.base_lr\n",
    "        self.opt.lr = lr\n",
    "        self.lrs.append(lr)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "onecyc = OneCycle(0.1)\n",
    "learn = Learner(simple_cnn(), dls, cross_entropy, lr=0.1, cbs=cbs+[onecyc])"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "learn.fit(8)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "plt.plot(onecyc.lrs);"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Conclusion"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Questionnaire"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "> tip: Experiments: For the questions here that ask you to explain what some function or class is, you should also complete your own code experiments."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "1. What is `glob`?\n",
    "1. How do you open an image with the Python imaging library?\n",
    "1. What does `L.map` do?\n",
    "1. What does `Self` do?\n",
    "1. What is `L.val2idx`?\n",
    "1. What methods do you need to implement to create your own `Dataset`?\n",
    "1. Why do we call `convert` when we open an image from Imagenette?\n",
    "1. What does `~` do? How is it useful for splitting training and validation sets?\n",
    "1. Does `~` work with the `L` or `Tensor` classes? What about NumPy arrays, Python lists, or pandas DataFrames?\n",
    "1. What is `ProcessPoolExecutor`?\n",
    "1. How does `L.range(self.ds)` work?\n",
    "1. What is `__iter__`?\n",
    "1. What is `first`?\n",
    "1. What is `permute`? Why is it needed?\n",
    "1. What is a recursive function? How does it help us define the `parameters` method?\n",
    "1. Write a recursive function that returns the first 20 items of the Fibonacci sequence.\n",
    "1. What is `super`?\n",
    "1. Why do subclasses of `Module` need to override `forward` instead of defining `__call__`?\n",
    "1. In `ConvLayer`, why does `init` depend on `act`?\n",
    "1. Why does `Sequential` need to call `register_modules`?\n",
    "1. Write a hook that prints the shape of every layer's activations.\n",
    "1. What is \"LogSumExp\"?\n",
    "1. Why is `log_softmax` useful?\n",
    "1. What is `GetAttr`? How is it helpful for callbacks?\n",
    "1. Reimplement one of the callbacks in this chapter without inheriting from `Callback` or `GetAttr`.\n",
    "1. What does `Learner.__call__` do?\n",
    "1. What is `getattr`? (Note the case difference to `GetAttr`!)\n",
    "1. Why is there a `try` block in `fit`?\n",
    "1. Why do we check for `model.training` in `one_batch`?\n",
    "1. What is `store_attr`?\n",
    "1. What is the purpose of `TrackResults.before_epoch`?\n",
    "1. What does `model.cuda` do? How does it work?\n",
    "1. Why do we need to check `model.training` in `LRFinder` and `OneCycle`?\n",
    "1. Use cosine annealing in `OneCycle`."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Further Research"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "1. Write `resnet18` from scratch (refer to <<chapter_resnet>> as needed), and train it with the `Learner` in this chapter.\n",
    "1. Implement a batchnorm layer from scratch and use it in your `resnet18`.\n",
    "1. Write a Mixup callback for use in this chapter.\n",
    "1. Add momentum to SGD.\n",
    "1. Pick a few features that you're interested in from fastai (or any other library) and implement them in this chapter.\n",
    "1. Pick a research paper that's not yet implemented in fastai or PyTorch and implement it in this chapter.\n",
    "  - Port it over to fastai.\n",
    "  - Submit a pull request to fastai, or create your own extension module and release it. \n",
    "  - Hint: you may find it helpful to use [`nbdev`](https://nbdev.fast.ai/) to create and deploy your package."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
  }
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