前言
[NLP]——The Annotated Transformer（模型篇）
One example
Tools
Data
class Batch:
"Object for holding a batch of data with mask during training."
def __init__(self, src, trg=None, pad=0):
self.src = src
self.src_mask = (src != pad).unsqueeze(-2)
if trg is not None:
self.trg = trg[:, :-1] # 通过前一个位置
self.trg_y = trg[:, 1:] # 预测后一个位置
self.trg_mask = \
self.make_std_mask(self.trg, pad)
self.ntokens = (self.trg_y != pad).data.sum()
@staticmethod
def make_std_mask(tgt, pad):
"Create a mask to hide padding and future words."
tgt_mask = (tgt != pad).unsqueeze(-2) # 长度pad
tgt_mask = tgt_mask & Variable(
subsequent_mask(tgt.size(-1)).type_as(tgt_mask.data)) # 长度pad & mask
return tgt_mask
global max_src_in_batch, max_tgt_in_batch
def batch_size_fn(new, count, sofar):
"Keep augmenting batch and calculate total number of tokens + padding."
global max_src_in_batch, max_tgt_in_batch
if count == 1:
max_src_in_batch = 0
max_tgt_in_batch = 0
max_src_in_batch = max(max_src_in_batch, len(new.src))
max_tgt_in_batch = max(max_tgt_in_batch, len(new.trg) + 2)
src_elements = count * max_src_in_batch
tgt_elements = count * max_tgt_in_batch
return max(src_elements, tgt_elements)
def data_gen(V, batch, nbatches):
"Generate random data for a src-tgt copy task."
for i in range(nbatches):
data = torch.from_numpy(np.random.randint(1, V, size=(batch, 10)))
data[:, 0] = 1 # 1表示start，后面decode时，也是先从1开始
src = Variable(data, requires_grad=False)
tgt = Variable(data, requires_grad=False)
yield Batch(src, tgt, 0)
LOSS & OPTIM
计算学习率的公式:
l r a t e = d m o d e l − 0.5 ∗ m i n ( s t e p − 0.5 , s t e p ∗ w a r m u p − 1.5 ) l_{rate} = d_{model}^{-0.5} * min(step^{-0.5}, step * warmup ^{-1.5}) lrate=dmodel−0.5∗min(step−0.5,step∗warmup−1.5)
class NoamOpt:
"Optim wrapper that implements rate."
def __init__(self, model_size, factor, warmup, optimizer):
self.optimizer = optimizer
self._step = 0
self.warmup = warmup
self.factor = factor
self.model_size = model_size
self._rate = 0
def step(self):
"Update parameters and rate"
self._step += 1
rate = self.rate()
for p in self.optimizer.param_groups:
p['lr'] = rate
self._rate = rate
self.optimizer.step()
def rate(self, step = None): # 计算每一个step的学习率
"Implement `lrate` above"
if step is None:
step = self._step
return self.factor * \
(self.model_size ** (-0.5) *
min(step ** (-0.5), step * self.warmup ** (-1.5)))
趋势：一开始 s t e p ∗ w a r m u p − 1.5 step * warmup ^ {-1.5} step∗warmup−1.5更小，所以随着step而线性增长，后来随着step增加， s t e p − 0.5 step^{-0.5} step−0.5的减小，而减小。
opts = [NoamOpt(512, 1, 4000, None),
NoamOpt(512, 1, 8000, None),
NoamOpt(256, 1, 4000, None)]
plt.plot(np.arange(1, 20000), [[opt.rate(i) for opt in opts] for i in range(1, 20000)])
plt.legend(["512:4000", "512:8000", "256:4000"])
def get_std_opt(model):
return NoamOpt(model.src_embed[0].d_model, 2, 4000,
torch.optim.Adam(model.parameters(), lr=0, betas=(0.9, 0.98), eps=1e-9))
class SimpleLossCompute:
"A simple loss compute and train function."
def __init__(self, generator, criterion, opt=None):
self.generator = generator # 模型中的分类层
self.criterion = criterion # KL散度定义的loss
self.opt = opt # 学习率调节器
def __call__(self, x, y, norm):
x = self.generator(x)
loss = self.criterion(x.contiguous().view(-1, x.size(-1)),
y.contiguous().view(-1)) / norm
loss.backward()
if self.opt is not None:
self.opt.step()
self.opt.optimizer.zero_grad()
return loss.data[0] * norm
Regularization
class LabelSmoothing(nn.Module):
"Implement label smoothing."
def __init__(self, size, padding_idx, smoothing=0.0):
super(LabelSmoothing, self).__init__()
self.criterion = nn.KLDivLoss(size_average=False)
self.padding_idx = padding_idx
self.confidence = 1.0 - smoothing
self.smoothing = smoothing
self.size = size
self.true_dist = None
def forward(self, x, target):
assert x.size(1) == self.size
true_dist = x.data.clone()
true_dist.fill_(self.smoothing / (self.size - 2)) # 先用 smoothing / size -2 填满整个向量
true_dist.scatter_(1, target.data.unsqueeze(1), self.confidence) # 再在target所对应的下标位置，填上置信度
true_dist[:, self.padding_idx] = 0
mask = torch.nonzero(target.data == self.padding_idx)
if mask.dim() > 0:
true_dist.index_fill_(0, mask.squeeze(), 0.0)
self.true_dist = true_dist
return self.criterion(x, Variable(true_dist, requires_grad=False))
crit = LabelSmoothing(5, 0, 0.4)
predict = torch.FloatTensor([[0, 0.2, 0.7, 0.1, 0], # 这里的0理解为padding，不计入loss
[0, 0.2, 0.7, 0.1, 0],
[0, 0.2, 0.7, 0.1, 0]])
v = crit(Variable(predict.log()),
Variable(torch.LongTensor([2, 1, 0]))) # 这里的0也代表padding?
# Show the target distributions expected by the system.
plt.imshow(crit.true_dist)
KL散度衡量的是两个概率分布之间的相似性，公式如下:
D K L ( p ∣ ∣ q ) = ∑ i = 1 N p ( x i ) log p ( x i ) / q ( x i ) D_{KL}(p||q) = \sum_{i=1}^{N}p(x_i)\log{p(x_i)/q(x_i)} DKL(p∣∣q)=i=1∑Np(xi)logp(xi)/q(xi)
但下面的代码表明，如果相似性到达一定程度，反而会有惩罚？？？(原文说的是:Lobel smoothing actually starts to penalize the model if it gets very confident about a given choice.)
crit = LabelSmoothing(5, 0, 0.1)
def loss(x):
d = x + 3 * 1
predict = torch.FloatTensor([[0, x / d, 1 / d, 1 / d, 1 / d],
])
#print(predict)
return crit(Variable(predict.log()),
Variable(torch.LongTensor([1]))).data[0]
plt.plot(np.arange(1, 100), [loss(x) for x in range(1, 100)])
RUN
def run_epoch(data_iter, model, loss_compute):
"Standard Training and Logging Function"
start = time.time()
total_tokens = 0
total_loss = 0
tokens = 0
for i, batch in enumerate(data_iter):
out = model.forward(batch.src, batch.trg,
batch.src_mask, batch.trg_mask)
loss = loss_compute(out, batch.trg_y, batch.ntokens)
total_loss += loss
total_tokens += batch.ntokens
tokens += batch.ntokens
if i % 50 == 1:
elapsed = time.time() - start
print("Epoch Step: %d Loss: %f Tokens per Sec: %f" %
(i, loss / batch.ntokens, tokens / elapsed))
start = time.time()
tokens = 0
return total_loss / total_tokens
训练
# Train the simple copy task.
V = 11
criterion = LabelSmoothing(size=V, padding_idx=0, smoothing=0.0)
model = make_model(V, V, N=2)
model_opt = NoamOpt(model.src_embed[0].d_model, 1, 400,
torch.optim.Adam(model.parameters(), lr=0, betas=(0.9, 0.98), eps=1e-9))
for epoch in range(10):
model.train()
run_epoch(data_gen(V, 30, 20), model,
SimpleLossCompute(model.generator, criterion, model_opt))
model.eval()
print(run_epoch(data_gen(V, 30, 5), model,
SimpleLossCompute(model.generator, criterion, None)))
测试
def greedy_decode(model, src, src_mask, max_len, start_symbol):
memory = model.encode(src, src_mask)
ys = torch.ones(1, 1).fill_(start_symbol).type_as(src.data)
for i in range(max_len-1):
out = model.decode(memory, src_mask,
Variable(ys),
Variable(subsequent_mask(ys.size(1))
.type_as(src.data)))
prob = model.generator(out[:, -1])
_, next_word = torch.max(prob, dim = 1)
next_word = next_word.data[0]
ys = torch.cat([ys,
torch.ones(1, 1).type_as(src.data).fill_(next_word)], dim=1)
return ys
model.eval()
src = Variable(torch.LongTensor([[1,2,3,4,5,6,7,8,9,10]]) )
src_mask = Variable(torch.ones(1, 1, 10) )
print(greedy_decode(model, src, src_mask, max_len=10, start_symbol=1))
Real World Example
# For data loading.
from torchtext import data, datasets
if True:
import spacy
spacy_de = spacy.load('de')
spacy_en = spacy.load('en')
def tokenize_de(text):
return [tok.text for tok in spacy_de.tokenizer(text)]
def tokenize_en(text):
return [tok.text for tok in spacy_en.tokenizer(text)]
BOS_WORD = '<s>'
EOS_WORD = '</s>'
BLANK_WORD = "<blank>"
SRC = data.Field(tokenize=tokenize_de, pad_token=BLANK_WORD)
TGT = data.Field(tokenize=tokenize_en, init_token = BOS_WORD,
eos_token = EOS_WORD, pad_token=BLANK_WORD)
MAX_LEN = 100
train, val, test = datasets.IWSLT.splits(
exts=('.de', '.en'), fields=(SRC, TGT),
filter_pred=lambda x: len(vars(x)['src']) <= MAX_LEN and
len(vars(x)['trg']) <= MAX_LEN)
MIN_FREQ = 2
SRC.build_vocab(train.src, min_freq=MIN_FREQ)
TGT.build_vocab(train.trg, min_freq=MIN_FREQ)
妈的spacy下载不下来