def TransformerLM(vocab_size, # pylint: disable=invalid-name
mode='train',
num_layers=6,
feature_depth=512,
feedforward_depth=2048,
num_heads=8,
dropout=0.9,
max_len=256):
"""Transformer language model (only uses the decoder part of Transformer).
Args:
vocab_size: int: vocab size
mode: str: 'train' or 'eval'
num_layers: int: number of encoder/decoder layers
feature_depth: int: depth of embedding
feedforward_depth: int: depth of feed-forward layer
num_heads: int: number of attention heads
dropout: float: dropout rate - Stax follows TF's KEEP probability convention
max_len: int: maximum symbol length for positional encoding
Returns:
init and apply.
"""
# Multi-headed Attention and Feed-forward layers
multi_attention = stax.MultiHeadedAttention(
feature_depth, num_heads=num_heads, dropout=dropout, mode=mode)
feed_forward = stax.serial(
stax.Dense(feedforward_depth, W_init=stax.xavier_uniform()),
stax.Relu,
stax.Dropout(dropout, mode=mode),
stax.Dense(feature_depth, W_init=stax.xavier_uniform())
)
# Single decoder layer
decoder_layer = stax.serial(
# target attends to self
stax.residual(stax.LayerNorm(feature_depth),
stax.multiplex(stax.Identity, # query
stax.Identity, # key
stax.Identity, # value
stax.CausalMask(axis=-2)), # attention mask
multi_attention,
stax.Dropout(dropout, mode=mode)),
# feed-forward
stax.residual(stax.LayerNorm(feature_depth),
feed_forward,
stax.Dropout(dropout, mode=mode))
)
return stax.serial(
stax.ShiftRight(),
stax.Embedding(feature_depth, vocab_size),
stax.PositionalEncoding(feature_depth, max_len=max_len),
stax.Dropout(dropout, mode=mode),
stax.repeat(decoder_layer, num_layers),
stax.LayerNorm(feature_depth),
stax.Dense(vocab_size, W_init=stax.xavier_uniform()),
stax.LogSoftmax
)
def TransformerLM(vocab_size,
feature_depth=512,
feedforward_depth=2048,
num_layers=6,
num_heads=8,
dropout=0.1,
max_len=2048,
mode='train'):
"""Transformer language model (only uses the decoder part of Transformer).
Args:
vocab_size: int: vocab size
feature_depth: int: depth of embedding
feedforward_depth: int: depth of feed-forward layer
num_layers: int: number of encoder/decoder layers
num_heads: int: number of attention heads
dropout: float: dropout rate (how much to drop out)
max_len: int: maximum symbol length for positional encoding
mode: str: 'train' or 'eval'
Returns:
init and apply.
"""
return stax.serial(
stax.ShiftRight(),
stax.Embedding(feature_depth, vocab_size),
stax.Dropout(dropout, mode=mode),
stax.PositionalEncoding(feature_depth, max_len=max_len),
stax.repeat(
DecoderLayer(
feature_depth, feedforward_depth, num_heads, dropout, mode),
num_layers),
stax.LayerNorm(),
stax.Dense(vocab_size, W_init=stax.xavier_uniform()),
stax.LogSoftmax
)
def Transformer(source_vocab_size, # pylint: disable=invalid-name
target_vocab_size,
mode='train',
num_layers=6,
feature_depth=512,
feedforward_depth=2048,
num_heads=8,
dropout=0.9,
shared_embedding=True,
max_len=200,
return_evals=False):
"""Transformer model.
Args:
source_vocab_size: int: source vocab size
target_vocab_size: int: target vocab size
mode: str: 'train' or 'eval'
num_layers: int: number of encoder/decoder layers
feature_depth: int: depth of embedding
feedforward_depth: int: depth of feed-forward layer
num_heads: int: number of attention heads
dropout: float: dropout rate - Stax follows TF's KEEP probability convention
shared_embedding: bool: specify whether source/target embeddings are tied.
max_len: int: maximum symbol length for positional encoding
return_evals: bool: whether to generate decode-time evaluation functions
Returns:
A namedtuple containing model 'init' and 'apply' functions for training and
the 'evals' functions that itself returns a namedtuple containing evaluation
functions for the trained encoder, decoder, and generator substax.
"""
# Input embedding and positional encoding
inject_position = stax.serial(
stax.PositionalEncoding(feature_depth, max_len=max_len),
stax.Dropout(dropout, mode=mode)
)
if shared_embedding:
assert source_vocab_size == target_vocab_size
# Weight-shared Embedding
embedding = stax.Share(stax.Embedding(feature_depth, source_vocab_size))
source_embedding_layer = stax.serial(embedding, inject_position)
target_embedding_layer = source_embedding_layer
else:
source_embedding = stax.Embedding(feature_depth, source_vocab_size)
target_embedding = stax.Embedding(feature_depth, target_vocab_size)
source_embedding_layer = stax.serial(source_embedding, inject_position)
target_embedding_layer = stax.serial(target_embedding, inject_position)
# Multi-headed Attention and Feed-forward layers
multi_attention = stax.MultiHeadedAttention(
feature_depth, num_heads=num_heads, dropout=dropout, mode=mode)
feed_forward = stax.serial(
stax.Dense(feedforward_depth, W_init=stax.xavier_uniform()),
stax.Relu,
stax.Dropout(dropout, mode=mode),
stax.Dense(feature_depth, W_init=stax.xavier_uniform())
)
# Encoder
@stax.Lambda
def encoder(source, source_mask):
"""Transformer encoder stack.
Args:
source: staxlayer variable: raw source sequences
source_mask: staxlayer variable: self-attention mask
Returns:
Staxlayer variable that outputs encoded source.
"""
encoder_layer = stax.serial(
# input attends to self
stax.residual(stax.LayerNorm(feature_depth),
stax.multiplex(stax.Identity, # query
stax.Identity, # key
stax.Identity, # value
source_mask), # attention mask
multi_attention,
stax.Dropout(dropout, mode=mode)),
# feed-forward
stax.residual(stax.LayerNorm(feature_depth),
feed_forward,
stax.Dropout(dropout, mode=mode))
)
return stax.serial(
source,
source_embedding_layer,
stax.repeat(encoder_layer, num_layers),
stax.LayerNorm(feature_depth),
)
# Decoder
@stax.Lambda
def decoder(memory, target, target_mask, memory_mask):
"""Transformer decoder stack.
Args:
memory: staxlayer variable: encoded source sequences
target: staxlayer variable: raw target sequences
target_mask: staxlayer variable: self-attention mask
memory_mask: staxlayer variable: memory attention mask
Returns:
Staxlayer variable that outputs encoded source.
"""
decoder_layer = stax.serial(
# target attends to self
stax.residual(stax.LayerNorm(feature_depth),
stax.multiplex(stax.Identity, # query
stax.Identity, # key
stax.Identity, # value
target_mask), # attention mask
multi_attention,
stax.Dropout(dropout, mode=mode)),
# target attends to encoded source
stax.residual(stax.LayerNorm(feature_depth),
stax.multiplex(stax.Identity, # query
memory, # key
memory, # value
memory_mask), # attention mask
multi_attention,
stax.Dropout(dropout, mode=mode)),
# feed-forward
stax.residual(stax.LayerNorm(feature_depth),
feed_forward,
stax.Dropout(dropout, mode=mode))
)
return stax.serial(
target,
target_embedding_layer,
stax.repeat(decoder_layer, num_layers),
stax.LayerNorm(feature_depth),
)
# The Transformer
@stax.Lambda
def transformer(source, target, source_mask, target_mask, memory_mask):
encoded_source = encoder(source, source_mask)
return decoder(encoded_source, target, target_mask, memory_mask)
# Finally, bind the generator transform to use later for inference.
@stax.Lambda
def generator(encoded_target):
return stax.serial(
encoded_target,
stax.Dense(target_vocab_size, W_init=stax.xavier_uniform()),
stax.LogSoftmax
)
# Model-Building and Evaluation Functions
# Get entire model's init and apply pair
top_init, top_apply = generator(transformer)
# By default act as a normal Stax constructor and emit an (init, apply) pair.
if not return_evals:
return (top_init, top_apply)
else:
# Inference-time function for binding trained params to model and returning
# the python-bound sub-expressions for evaluation and sequence generation.
def make_namedtuple(**kwargs):
return collections.namedtuple('Model', kwargs.keys())(**kwargs)
def get_evals(params):
# We need to feed _concrete_ trained parameters through the network once.
# Otherwise the bound parameters point to abstract tracer values.
# The inputs don't matter.
fake_inputs = 5 * (np.ones((1), dtype=np.int32),)
fake_key = random.PRNGKey(1)
top_apply(params, fake_inputs, rng=fake_key)
# We can now return eval functions from the bound pieces of the model.
return make_namedtuple(
encoder=stax.make_apply_fun(encoder),
generator=stax.make_apply_fun(generator),
decoder=stax.make_apply_fun(decoder),
)
# We return the functions needed to train and evaluate the Transformer.
return make_namedtuple(
init=top_init,
apply=top_apply,
evals=get_evals,
)
def Transformer(source_vocab_size,
target_vocab_size,
mode='train',
num_layers=6,
feature_depth=512,
feedforward_depth=2048,
num_heads=8,
dropout=0.1,
shared_embedding=True,
max_len=200,
return_evals=False):
"""Transformer model.
Args:
source_vocab_size: int: source vocab size
target_vocab_size: int: target vocab size
mode: str: 'train' or 'eval'
num_layers: int: number of encoder/decoder layers
feature_depth: int: depth of embedding
feedforward_depth: int: depth of feed-forward layer
num_heads: int: number of attention heads
dropout: float: dropout rate (how much to drop out)
shared_embedding: bool: specify whether source/target embeddings are tied.
max_len: int: maximum symbol length for positional encoding
return_evals: bool: whether to generate decode-time evaluation functions
Returns:
A namedtuple containing model 'init' and 'apply' functions for training and
the 'evals' functions that itself returns a namedtuple containing evaluation
functions for the trained encoder, decoder, and generator substax.
"""
# Input embedding and positional encoding
inject_position = stax.serial(
stax.Dropout(dropout, mode=mode),
stax.PositionalEncoding(feature_depth, max_len=max_len)
)
if shared_embedding:
assert source_vocab_size == target_vocab_size
# Weight-shared Embedding
embedding = stax.Share(stax.Embedding(feature_depth, source_vocab_size))
source_embedding_layer = stax.serial(embedding, inject_position)
target_embedding_layer = source_embedding_layer
else:
source_embedding = stax.Embedding(feature_depth, source_vocab_size)
target_embedding = stax.Embedding(feature_depth, target_vocab_size)
source_embedding_layer = stax.serial(source_embedding, inject_position)
target_embedding_layer = stax.serial(target_embedding, inject_position)
# Multi-headed Attention and Feed-forward layers
multi_attention = stax.MultiHeadedAttention(
feature_depth, num_heads=num_heads, dropout=dropout, mode=mode)
# Encoder
@stax.Lambda
def Encoder(source, source_mask):
"""Transformer encoder stack.
Args:
source: staxlayer variable: raw source sequences
source_mask: staxlayer variable: self-attention mask
Returns:
Staxlayer variable that outputs encoded source.
"""
encoder_layer = stax.serial(
# input attends to self
stax.residual(stax.LayerNorm(),
stax.FanOut(4),
stax.parallel(stax.Identity, # query
stax.Identity, # key
stax.Identity, # value
source_mask), # attention mask
multi_attention,
stax.Dropout(dropout, mode=mode)),
# feed-forward
ResidualFeedForward(
feature_depth, feedforward_depth, dropout, mode=mode),
)
return stax.serial(
source,
source_embedding_layer,
stax.repeat(encoder_layer, num_layers),
stax.LayerNorm(),
)
# Decoder
@stax.Lambda
def Decoder(memory, target, target_mask, memory_mask):
"""Transformer decoder stack.
Args:
memory: staxlayer variable: encoded source sequences
target: staxlayer variable: raw target sequences
target_mask: staxlayer variable: self-attention mask
memory_mask: staxlayer variable: memory attention mask
Returns:
Staxlayer variable that outputs encoded source.
"""
decoder_layer = stax.serial(
# target attends to self
stax.residual(stax.LayerNorm(),
stax.FanOut(4),
stax.parallel(stax.Identity, # query
stax.Identity, # key
stax.Identity, # value
target_mask), # attention mask
multi_attention,
stax.Dropout(dropout, mode=mode)),
# target attends to encoded source
stax.residual(stax.LayerNorm(),
stax.FanOut(4),
stax.parallel(stax.Identity, # query
memory, # key
memory, # value
memory_mask), # attention mask
multi_attention,
stax.Dropout(dropout, mode=mode)),
# feed-forward
ResidualFeedForward(
feature_depth, feedforward_depth, dropout, mode=mode)
)
return stax.serial(
target,
target_embedding_layer,
stax.repeat(decoder_layer, num_layers),
stax.LayerNorm(),
)
# The Transformer
@stax.Lambda
def transformer(source, target, source_mask, target_mask, memory_mask): # pylint: disable=invalid-name
encoded_source = Encoder(source, source_mask)
return Decoder(encoded_source, target, target_mask, memory_mask)
# Finally, bind the generator transform to use later for inference.
@stax.Lambda
def Generator(encoded_target):
return stax.serial(
encoded_target,
stax.Dense(target_vocab_size, W_init=stax.xavier_uniform()),
stax.LogSoftmax
)
# Model-Building and Evaluation Functions
# Get entire model's init and apply pair
top_init, top_apply = Generator(transformer)
# By default act as a normal Stax constructor and emit an (init, apply) pair.
if not return_evals:
return (top_init, top_apply)
else:
raise ValueError('inference in this model is still a work in progress')