def SRU(n_units, activation=None):
"""SRU layer as in https://arxiv.org/abs/1709.02755.
As defined in the paper:
(1) y_t = W x_t (+ B optionally, which we do)
(2) f_t = sigmoid(Wf x_t + bf)
(3) r_t = sigmoid(Wr x_t + br)
(4) c_t = f_t * c_{t-1} + (1 - f_t) * y_t
(5) h_t = r_t * activation(c_t) + (1 - r_t) * x_t
We assume the input is of shape [batch, length, depth] and recurrence
happens on the length dimension. This returns a single layer. It's best
to use at least 2, they say in the paper, except inside a Transformer.
Args:
n_units: output depth of the SRU layer.
activation: Optional activation function.
Returns:
The SRU layer.
"""
activation = activation or []
return cb.Serial(
cb.Dup(), # x, x
core.Dense(3 * n_units),
cb.Split(n_items=3), # r, f, y, x
cb.Parallel(core.Sigmoid(), core.Sigmoid()), # r, f, y, x
cb.Fn(lambda r, f, y: (y * (1.0 - f), f, r)), # y * (1 - f), f, r, x
cb.Parallel([], [], [cb.Dup(), MakeZeroState()]), # pylint: disable=no-value-for-parameter
cb.Scan(InnerSRUCell(), axis=1), # pylint: disable=no-value-for-parameter
cb.Parallel(activation, cb.Drop()), # act(c), r, x
cb.Fn(lambda c, r, x: c * r + x * (1 - r)))
def test_fn_layer_varargs_n_in(self):
with self.assertRaisesRegexp(ValueError, 'variable arg'):
cb.Fn(lambda *args: args[0])
# Check that varargs work when n_in is set.
id_layer = cb.Fn(lambda *args: args[0], n_in=1)
input_signature = ShapeDtype((2, 7))
expected_shape = (2, 7)
output_shape = base.check_shape_agreement(id_layer, input_signature)
self.assertEqual(output_shape, expected_shape)
def PositionalEmbeddings(d_feature, separate_cls, total_kv_pooling):
"""Positional embedding for relative attention.
Returns a layer that based on queries, keys and accumulated pool size of
keys/values until this layer calculates sinusoidal positional embeddings
for relative attention calculations.
Args:
d_feature: Depth/dimensionality of feature embedding.
separate_cls: True/False if we separate_cls in calculations.
total_kv_pooling: Accumulated pool size of keys/values until this layer.
Returns:
Positional embedding.
"""
def PositionsVectors(queries, keys):
is_funnel_layer = queries.shape != keys.shape
keys_len, queries_len = keys.shape[1], queries.shape[1]
current_pooling_ratio = keys_len / queries_len
# Special case of upsampling
if is_funnel_layer and current_pooling_ratio < 1:
# We should not be doing standard upsampling when we use separate_cls
# Cls token is being used for classification
assert not separate_cls
assert (total_kv_pooling * keys_len) % queries_len == 0
multiplier = ((total_kv_pooling * keys_len) // queries_len)
positions = jnp.arange(-queries_len + 1, queries_len, 1.0) * multiplier
else:
positions = jnp.arange(-keys_len + 1, keys_len, 1.0) * total_kv_pooling
if is_funnel_layer and separate_cls:
# For pool_size 2 without separating cls we have got
# [0][1][2][3][4][5][6][7] -> [01][23][45][67]
# With separating cls we have got
# [0][1][2][3][4][5][6][7] -> [0][12][34][56]
# First group always will always consist of one token after pooling
# instead of (pool_size) tokens. We need to add proper offset so
# that our shift later on in calculating attention works properly
cls_offset = (current_pooling_ratio - 1) * total_kv_pooling
positions = positions + cls_offset
return positions
def Sinusoidal_Embeddings(positions):
inv_freq = 1 / (10000**(jnp.arange(0.0, d_feature, 2.0) / d_feature))
sinusoid_freq = jnp.einsum('i,j->ij', positions, inv_freq)
pos_emb = jnp.concatenate(
[jnp.sin(sinusoid_freq), jnp.cos(sinusoid_freq)], axis=1)
return pos_emb
return cb.Serial(
cb.Fn('Generate positions vectors', PositionsVectors, n_out=1),
cb.Fn(
'Transform to sinusoidal encodings', Sinusoidal_Embeddings, n_out=1))
def test_fn_layer_difficult_n_out(self):
with self.assertRaisesRegexp(ValueError, 'n_out'):
# Determining the output of this layer is hard with dummies.
cb.Fn(lambda x: np.concatencate([x, x], axis=4))
# Check that this layer works when n_out is set.
layer = cb.Fn(lambda x: np.concatenate([x, x], axis=4), n_out=1)
input_signature = ShapeDtype((2, 1, 2, 2, 3))
expected_shape = (2, 1, 2, 2, 6)
output_shape = base.check_shape_agreement(layer, input_signature)
self.assertEqual(output_shape, expected_shape)
def PositionalEmbeddings(d_feature, separate_cls, total_kv_pooling):
"""Positional embeddings.
Args:
d_feature: Depth/dimensionality of feature embedding.
separate_cls: True/False if we separate_cls in calculations.
total_kv_pooling: Accumulated pool size of keys/values until this layer.
Returns:
a layer that based on queries, keys and accumulated pool size of
keys/values until this layer calculates sinusoidal positional embeddings
for relative attention calculations.
"""
def PositionsVectors(queries, keys):
assert not separate_cls
keys_len, queries_len = keys.shape[-2], queries.shape[-2]
funnel_factor, is_upsampling = calc_funnel_ratio(keys_len, queries_len)
if funnel_factor == 1:
offset = keys_len - 1
positions = (jnp.arange(keys_len) - offset) * total_kv_pooling
else:
if is_upsampling:
positions = jnp.arange(-queries_len + 1, queries_len, 1.0)
else:
positions = jnp.arange(-keys_len + 1, keys_len,
1.0) * total_kv_pooling
return positions
def Sinusoidal_Embeddings(positions):
inv_freq = 1 / (10000**(jnp.arange(0.0, d_feature, 2.0) / d_feature))
sinusoid_freq = jnp.einsum('i,j->ij', positions, inv_freq)
pos_emb = jnp.concatenate(
[jnp.sin(sinusoid_freq),
jnp.cos(sinusoid_freq)], axis=1)
return pos_emb
return cb.Serial(
cb.Fn('Generate positions vectors', PositionsVectors, n_out=1),
cb.Fn('Transform to sinusoidal encodings',
Sinusoidal_Embeddings,
n_out=1))
def test_fn_layer_example(self):
layer = cb.Fn(lambda x, y: (x + y, np.concatenate([x, y], axis=0)))
input_signature = (ShapeDtype((2, 7)), ShapeDtype((2, 7)))
expected_shape = ((2, 7), (4, 7))
output_shape = base.check_shape_agreement(layer, input_signature)
self.assertEqual(output_shape, expected_shape)
inp = (np.array([2]), np.array([3]))
x, xs = layer(inp)
self.assertEqual(int(x), 5)
self.assertEqual([int(y) for y in xs], [2, 3])
def ShiftRightCls(cls_id):
"""Shifts right.
Returns a layer that shifts input tokens to the right by one
and inserts an cls token to the beginning like in BERT paper.
Args:
cls_id: id of the cls token in embedding dictionary.
Returns:
shift_right layer.
"""
def shift_right(x):
pad_widths = [(0, 0)] * len(x.shape)
pad_widths[1] = (1, 0)
padded = jnp.pad(
x, pad_widths, mode='constant', constant_values=x.dtype.type(cls_id))
return padded[:, :-1]
return cb.Fn('ShiftRightCls()', shift_right)
def CreateAttentionMaskLayer():
"""Creates attention mask layer.
Returns a layer that based on queries, keys and accumulated pool size of
keys/values until this layer calculates positional embeddings for
causal relative attention calculations.
Takes as input q, k, v and appends proper mask in the end.
Causal attention uses masking to prevent a given sequence position from
attending to positions greater than / following it. This is used, for
example, when training autoregressive sequence models, or when decoding a
sequence symbol by symbol.
Returns:
an attention mask layer.
"""
def calculate_mask(queries, keys):
batch_size = queries.shape[0]
keys_len, queries_len = keys.shape[-2], queries.shape[-2]
funnel_factor, is_upsampling = calc_funnel_ratio(keys_len, queries_len)
return _funnel_mask(batch_size, keys_len, queries_len, funnel_factor,
is_upsampling)
def _funnel_mask(batch_size, keys_len, queries_len, funnel_factor,
is_upsampling):
"""Funnel mask.
Args:
batch_size: batch size.
keys_len: keys length.
queries_len: queries length.
funnel_factor: funnel factor.
is_upsampling: True or False.
Returns:
funnel mask.
This function based on keys/queries lengths creates a triangle mask
that prevents tokens from attending to positions following it.
If funnel_factor is not equal to 1 due to funnel upsampling or
downsampling it adjusts created mask for funnel attention
by repeating each element funnel_factor times.
This is because after funnel layer one token attends to funnel_factor
different tokens in downsampling. During upsampling on the other hand
funnel_factor tokens are attending to single token before upsampling.
"""
if funnel_factor != 1:
if not is_upsampling:
mask = jnp.tril(
jnp.ones((queries_len, queries_len), dtype=jnp.bool_))
mask = jnp.repeat(mask, funnel_factor, axis=-1)
else:
mask = jnp.tril(jnp.ones((keys_len, keys_len),
dtype=jnp.bool_))
mask = jnp.repeat(mask, funnel_factor, axis=-2)
else:
mask = jnp.tril(
jnp.ones((queries_len, queries_len), dtype=jnp.bool_))
return jnp.repeat(mask[None, None, :, :], batch_size, axis=0)
return cb.Branch(
cb.Select([0]), cb.Select([1]), cb.Select([2]),
cb.Fn('create attention mask layer', calculate_mask, n_out=1))
def test_fn_layer_fails_wrong_f(self):
with self.assertRaisesRegexp(ValueError, 'default arg'):
cb.Fn(lambda x, sth=None: x)
with self.assertRaisesRegexp(ValueError, 'keyword arg'):
cb.Fn(lambda x, **kwargs: x)
