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{
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"# 预训练BERT\n",
":label:`sec_bert-pretraining`\n",
"\n",
"利用 :numref:`sec_bert`中实现的BERT模型和 :numref:`sec_bert-dataset`中从WikiText-2数据集生成的预训练样本,我们将在本节中在WikiText-2数据集上对BERT进行预训练。\n"
]
},
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"tab": [
"pytorch"
]
},
"outputs": [],
"source": [
"import torch\n",
"from torch import nn\n",
"from d2l import torch as d2l"
]
},
{
"cell_type": "markdown",
"id": "898d6f91",
"metadata": {
"origin_pos": 4
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"source": [
"首先,我们加载WikiText-2数据集作为小批量的预训练样本,用于遮蔽语言模型和下一句预测。批量大小是512,BERT输入序列的最大长度是64。注意,在原始BERT模型中,最大长度是512。\n"
]
},
{
"cell_type": "code",
"execution_count": 2,
"id": "95571e6a",
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"execution": {
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"source": [
"batch_size, max_len = 512, 64\n",
"train_iter, vocab = d2l.load_data_wiki(batch_size, max_len)"
]
},
{
"cell_type": "markdown",
"id": "cfb22b86",
"metadata": {
"origin_pos": 7
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"source": [
"## 预训练BERT\n",
"\n",
"原始BERT :cite:`Devlin.Chang.Lee.ea.2018`有两个不同模型尺寸的版本。基本模型($\\text{BERT}_{\\text{BASE}}$)使用12层(Transformer编码器块),768个隐藏单元(隐藏大小)和12个自注意头。大模型($\\text{BERT}_{\\text{LARGE}}$)使用24层,1024个隐藏单元和16个自注意头。值得注意的是,前者有1.1亿个参数,后者有3.4亿个参数。为了便于演示,我们定义了一个小的BERT,使用了2层、128个隐藏单元和2个自注意头。\n"
]
},
{
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"id": "3cc34825",
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"tab": [
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"source": [
"net = d2l.BERTModel(len(vocab), num_hiddens=128, norm_shape=[128],\n",
" ffn_num_input=128, ffn_num_hiddens=256, num_heads=2,\n",
" num_layers=2, dropout=0.2, key_size=128, query_size=128,\n",
" value_size=128, hid_in_features=128, mlm_in_features=128,\n",
" nsp_in_features=128)\n",
"devices = d2l.try_all_gpus()\n",
"loss = nn.CrossEntropyLoss()"
]
},
{
"cell_type": "markdown",
"id": "be063421",
"metadata": {
"origin_pos": 10
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"source": [
"在定义训练代码实现之前,我们定义了一个辅助函数`_get_batch_loss_bert`。给定训练样本,该函数计算遮蔽语言模型和下一句子预测任务的损失。请注意,BERT预训练的最终损失是遮蔽语言模型损失和下一句预测损失的和。\n"
]
},
{
"cell_type": "code",
"execution_count": 4,
"id": "64b2c84b",
"metadata": {
"execution": {
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"source": [
"#@save\n",
"def _get_batch_loss_bert(net, loss, vocab_size, tokens_X,\n",
" segments_X, valid_lens_x,\n",
" pred_positions_X, mlm_weights_X,\n",
" mlm_Y, nsp_y):\n",
" # 前向传播\n",
" _, mlm_Y_hat, nsp_Y_hat = net(tokens_X, segments_X,\n",
" valid_lens_x.reshape(-1),\n",
" pred_positions_X)\n",
" # 计算遮蔽语言模型损失\n",
" mlm_l = loss(mlm_Y_hat.reshape(-1, vocab_size), mlm_Y.reshape(-1)) *\\\n",
" mlm_weights_X.reshape(-1, 1)\n",
" mlm_l = mlm_l.sum() / (mlm_weights_X.sum() + 1e-8)\n",
" # 计算下一句子预测任务的损失\n",
" nsp_l = loss(nsp_Y_hat, nsp_y)\n",
" l = mlm_l + nsp_l\n",
" return mlm_l, nsp_l, l"
]
},
{
"cell_type": "markdown",
"id": "4e553304",
"metadata": {
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"source": [
"通过调用上述两个辅助函数,下面的`train_bert`函数定义了在WikiText-2`train_iter`)数据集上预训练BERT`net`)的过程。训练BERT可能需要很长时间。以下函数的输入`num_steps`指定了训练的迭代步数,而不是像`train_ch13`函数那样指定训练的轮数(参见 :numref:`sec_image_augmentation`)。\n"
]
},
{
"cell_type": "code",
"execution_count": 5,
"id": "6cd43502",
"metadata": {
"execution": {
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"tab": [
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"source": [
"def train_bert(train_iter, net, loss, vocab_size, devices, num_steps):\n",
" net = nn.DataParallel(net, device_ids=devices).to(devices[0])\n",
" trainer = torch.optim.Adam(net.parameters(), lr=0.01)\n",
" step, timer = 0, d2l.Timer()\n",
" animator = d2l.Animator(xlabel='step', ylabel='loss',\n",
" xlim=[1, num_steps], legend=['mlm', 'nsp'])\n",
" # 遮蔽语言模型损失的和,下一句预测任务损失的和,句子对的数量,计数\n",
" metric = d2l.Accumulator(4)\n",
" num_steps_reached = False\n",
" while step < num_steps and not num_steps_reached:\n",
" for tokens_X, segments_X, valid_lens_x, pred_positions_X,\\\n",
" mlm_weights_X, mlm_Y, nsp_y in train_iter:\n",
" tokens_X = tokens_X.to(devices[0])\n",
" segments_X = segments_X.to(devices[0])\n",
" valid_lens_x = valid_lens_x.to(devices[0])\n",
" pred_positions_X = pred_positions_X.to(devices[0])\n",
" mlm_weights_X = mlm_weights_X.to(devices[0])\n",
" mlm_Y, nsp_y = mlm_Y.to(devices[0]), nsp_y.to(devices[0])\n",
" trainer.zero_grad()\n",
" timer.start()\n",
" mlm_l, nsp_l, l = _get_batch_loss_bert(\n",
" net, loss, vocab_size, tokens_X, segments_X, valid_lens_x,\n",
" pred_positions_X, mlm_weights_X, mlm_Y, nsp_y)\n",
" l.backward()\n",
" trainer.step()\n",
" metric.add(mlm_l, nsp_l, tokens_X.shape[0], 1)\n",
" timer.stop()\n",
" animator.add(step + 1,\n",
" (metric[0] / metric[3], metric[1] / metric[3]))\n",
" step += 1\n",
" if step == num_steps:\n",
" num_steps_reached = True\n",
" break\n",
"\n",
" print(f'MLM loss {metric[0] / metric[3]:.3f}, '\n",
" f'NSP loss {metric[1] / metric[3]:.3f}')\n",
" print(f'{metric[2] / timer.sum():.1f} sentence pairs/sec on '\n",
" f'{str(devices)}')"
]
},
{
"cell_type": "markdown",
"id": "08640bff",
"metadata": {
"origin_pos": 18
},
"source": [
"在预训练过程中,我们可以绘制出遮蔽语言模型损失和下一句预测损失。\n"
]
},
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"text": [
"MLM loss 5.425, NSP loss 0.775\n",
"3485.7 sentence pairs/sec on [device(type='cuda', index=0), device(type='cuda', index=1)]\n"
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"source": [
"train_bert(train_iter, net, loss, len(vocab), devices, 50)"
]
},
{
"cell_type": "markdown",
"id": "ede604ea",
"metadata": {
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"source": [
"## 用BERT表示文本\n",
"\n",
"在预训练BERT之后,我们可以用它来表示单个文本、文本对或其中的任何词元。下面的函数返回`tokens_a`和`tokens_b`中所有词元的BERT`net`)表示。\n"
]
},
{
"cell_type": "code",
"execution_count": 7,
"id": "77f3b8e4",
"metadata": {
"execution": {
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"tab": [
"pytorch"
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},
"outputs": [],
"source": [
"def get_bert_encoding(net, tokens_a, tokens_b=None):\n",
" tokens, segments = d2l.get_tokens_and_segments(tokens_a, tokens_b)\n",
" token_ids = torch.tensor(vocab[tokens], device=devices[0]).unsqueeze(0)\n",
" segments = torch.tensor(segments, device=devices[0]).unsqueeze(0)\n",
" valid_len = torch.tensor(len(tokens), device=devices[0]).unsqueeze(0)\n",
" encoded_X, _, _ = net(token_ids, segments, valid_len)\n",
" return encoded_X"
]
},
{
"cell_type": "markdown",
"id": "25e0697e",
"metadata": {
"origin_pos": 25
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"source": [
"考虑“a crane is flying”这句话。回想一下 :numref:`subsec_bert_input_rep`中讨论的BERT的输入表示。插入特殊标记“&lt;cls&gt;”(用于分类)和“&lt;sep&gt;”(用于分隔)后,BERT输入序列的长度为6。因为零是“&lt;cls&gt;”词元,`encoded_text[:, 0, :]`是整个输入语句的BERT表示。为了评估一词多义词元“crane”,我们还打印出了该词元的BERT表示的前三个元素。\n"
]
},
{
"cell_type": "code",
"execution_count": 8,
"id": "1081fda9",
"metadata": {
"execution": {
"iopub.execute_input": "2023-08-18T07:05:00.673428Z",
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"shell.execute_reply": "2023-08-18T07:05:00.689347Z"
},
"origin_pos": 26,
"tab": [
"pytorch"
]
},
"outputs": [
{
"data": {
"text/plain": [
"(torch.Size([1, 6, 128]),\n",
" torch.Size([1, 128]),\n",
" tensor([-0.5007, -1.0034, 0.8718], device='cuda:0', grad_fn=<SliceBackward0>))"
]
},
"execution_count": 8,
"metadata": {},
"output_type": "execute_result"
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],
"source": [
"tokens_a = ['a', 'crane', 'is', 'flying']\n",
"encoded_text = get_bert_encoding(net, tokens_a)\n",
"# 词元:'<cls>','a','crane','is','flying','<sep>'\n",
"encoded_text_cls = encoded_text[:, 0, :]\n",
"encoded_text_crane = encoded_text[:, 2, :]\n",
"encoded_text.shape, encoded_text_cls.shape, encoded_text_crane[0][:3]"
]
},
{
"cell_type": "markdown",
"id": "203ca198",
"metadata": {
"origin_pos": 27
},
"source": [
"现在考虑一个句子“a crane driver came”和“he just left”。类似地,`encoded_pair[:, 0, :]`是来自预训练BERT的整个句子对的编码结果。注意,多义词元“crane”的前三个元素与上下文不同时的元素不同。这支持了BERT表示是上下文敏感的。\n"
]
},
{
"cell_type": "code",
"execution_count": 9,
"id": "960c3aa2",
"metadata": {
"execution": {
"iopub.execute_input": "2023-08-18T07:05:00.694637Z",
"iopub.status.busy": "2023-08-18T07:05:00.694061Z",
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"shell.execute_reply": "2023-08-18T07:05:00.707778Z"
},
"origin_pos": 28,
"tab": [
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},
"outputs": [
{
"data": {
"text/plain": [
"(torch.Size([1, 10, 128]),\n",
" torch.Size([1, 128]),\n",
" tensor([ 0.5101, -0.4041, -1.2749], device='cuda:0', grad_fn=<SliceBackward0>))"
]
},
"execution_count": 9,
"metadata": {},
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],
"source": [
"tokens_a, tokens_b = ['a', 'crane', 'driver', 'came'], ['he', 'just', 'left']\n",
"encoded_pair = get_bert_encoding(net, tokens_a, tokens_b)\n",
"# 词元:'<cls>','a','crane','driver','came','<sep>','he','just',\n",
"# 'left','<sep>'\n",
"encoded_pair_cls = encoded_pair[:, 0, :]\n",
"encoded_pair_crane = encoded_pair[:, 2, :]\n",
"encoded_pair.shape, encoded_pair_cls.shape, encoded_pair_crane[0][:3]"
]
},
{
"cell_type": "markdown",
"id": "d642486c",
"metadata": {
"origin_pos": 29
},
"source": [
"在 :numref:`chap_nlp_app`中,我们将为下游自然语言处理应用微调预训练的BERT模型。\n",
"\n",
"## 小结\n",
"\n",
"* 原始的BERT有两个版本,其中基本模型有1.1亿个参数,大模型有3.4亿个参数。\n",
"* 在预训练BERT之后,我们可以用它来表示单个文本、文本对或其中的任何词元。\n",
"* 在实验中,同一个词元在不同的上下文中具有不同的BERT表示。这支持BERT表示是上下文敏感的。\n",
"\n",
"## 练习\n",
"\n",
"1. 在实验中,我们可以看到遮蔽语言模型损失明显高于下一句预测损失。为什么?\n",
"2. 将BERT输入序列的最大长度设置为512(与原始BERT模型相同)。使用原始BERT模型的配置,如$\\text{BERT}_{\\text{LARGE}}$。运行此部分时是否遇到错误?为什么?\n"
]
},
{
"cell_type": "markdown",
"id": "9f6249ab",
"metadata": {
"origin_pos": 31,
"tab": [
"pytorch"
]
},
"source": [
"[Discussions](https://discuss.d2l.ai/t/5743)\n"
]
}
],
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