关于transformer模型总结（源码）
本文主要是对transfermer模型的源码进行解析：
transfermer主要结构是由encoder和decoder构成。其中，encoder是由embedding + positional_encoding作为输入，然后加一个dropout层，然后输入放到6个multihead_attention构成的结构中，每个multihead_attention后面跟一个feedforward。而decoder是由decoder embedding + positional_encoding作为输入，输入到dropout层，然后后面跟六个self multihead_attention+ multihead_attention，最后后面跟一个feedward。最后加一个liner projection。
1.主体模块介绍
1) positiion encoding
这个模块主要是对sequence中word的顺序进行编码，主要采用的是google论文中提出的公式。
def positional_encoding(inputs,
num_units,
zero_pad=True,
scale=True,
scope="positional_encoding",
reuse=None):
N, T = inputs.get_shape().as_list()
with tf.variable_scope(scope, reuse=reuse):
position_ind = tf.tile(tf.expand_dims(tf.range(T), 0), [tf.shape(inputs)[0], 1])
# First part of the PE function: sin and cos argument
position_enc = np.array([
[pos / np.power(10000, 2.*i/num_units) for i in range(num_units)]
for pos in range(T)])
# Second part, apply the cosine to even columns and sin to odds.
position_enc[:, 0::2] = np.sin(position_enc[:, 0::2]) # dim 2i
position_enc[:, 1::2] = np.cos(position_enc[:, 1::2]) # dim 2i+1
# Convert to a tensor
lookup_table = tf.convert_to_tensor(position_enc)
lookup_table = tf.cast(lookup_table,tf.float32)
if zero_pad:
lookup_table = tf.concat((tf.zeros(shape=[1, num_units]),
lookup_table[1:, :]), 0)
outputs = tf.nn.embedding_lookup(lookup_table, position_ind)
if scale:
outputs = outputs * num_units**0.5
return outputs
2）layer normalize
这个部分主要是对multi-head attention中的输出做layer normlaze，代码如下，与batch normalize相似，不过layer是横向的做normalize。
# layer normalize
def normalize(inputs,
epsilon = 1e-8,
scope="ln",
reuse=None):
with tf.variable_scope(scope, reuse=reuse):
inputs_shape = inputs.get_shape()
params_shape = inputs_shape[-1:]
mean, variance = tf.nn.moments(inputs, [-1], keep_dims=True)
beta= tf.Variable(tf.zeros(params_shape))
gamma = tf.Variable(tf.ones(params_shape))
normalized = (inputs - mean) / ( (variance + epsilon) ** (.5) )
outputs = gamma * normalized + beta
return outputs3）embedding
这个部分比较简单，是将语句中的每个词转化为向量
def embedding(inputs,
vocab_size,
num_units,
zero_pad=True,
scale=True,
scope="embedding",
reuse=None):
with tf.variable_scope(scope, reuse=reuse):
lookup_table = tf.get_variable('lookup_table',
dtype=tf.float32,
shape=[vocab_size, num_units],
initializer=tf.contrib.layers.xavier_initializer())
if zero_pad:
lookup_table = tf.concat((tf.zeros(shape=[1, num_units]),
lookup_table[1:, :]), 0)
outputs = tf.nn.embedding_lookup(lookup_table, inputs)
if scale:
outputs = outputs * (num_units ** 0.5)
return outputs4） label smoothing
对输入数据做平滑处理，这个部分不是太理解。
def label_smoothing(inputs, epsilon=0.1):
K = inputs.get_shape().as_list()[-1] # number of channels
return ((1-epsilon) * inputs) + (epsilon / K)5）multihead attention
5）multihead attention
def multihead_attention(queries,
keys,
num_units=None,
num_heads=8,
dropout_rate=0,
is_training=True,
causality=False,
scope="multihead_attention",
reuse=None):
with tf.variable_scope(scope, reuse=reuse):
# Set the fall back option for num_units
if num_units is None:
num_units = queries.get_shape().as_list[-1]
# Linear projections
Q = tf.layers.dense(queries, num_units, activation=tf.nn.relu) # (N, T_q, C)
K = tf.layers.dense(keys, num_units, activation=tf.nn.relu) # (N, T_k, C)
V = tf.layers.dense(keys, num_units, activation=tf.nn.relu) # (N, T_k, C)
# Split and concat
Q_ = tf.concat(tf.split(Q, num_heads, axis=2), axis=0) # (h*N, T_q, C/h)
K_ = tf.concat(tf.split(K, num_heads, axis=2), axis=0) # (h*N, T_k, C/h)
V_ = tf.concat(tf.split(V, num_heads, axis=2), axis=0) # (h*N, T_k, C/h)
# Multiplication
outputs = tf.matmul(Q_, tf.transpose(K_, [0, 2, 1])) # (h*N, T_q, T_k)
# Scale
outputs = outputs / (K_.get_shape().as_list()[-1] ** 0.5)
# Key Masking
key_masks = tf.sign(tf.abs(tf.reduce_sum(keys, axis=-1))) # (N, T_k)
key_masks = tf.tile(key_masks, [num_heads, 1]) # (h*N, T_k)
key_masks = tf.tile(tf.expand_dims(key_masks, 1), [1, tf.shape(queries)[1], 1]) # (h*N, T_q, T_k)
paddings = tf.ones_like(outputs)*(-2**32+1)
outputs = tf.where(tf.equal(key_masks, 0), paddings, outputs) # (h*N, T_q, T_k)
# Causality = Future blinding
if causality:
diag_vals = tf.ones_like(outputs[0, :, :]) # (T_q, T_k)
tril = tf.contrib.linalg.LinearOperatorLowerTriangular(diag_vals).to_dense() # (T_q, T_k)
masks = tf.tile(tf.expand_dims(tril, 0), [tf.shape(outputs)[0], 1, 1]) # (h*N, T_q, T_k)
paddings = tf.ones_like(masks)*(-2**32+1)
outputs = tf.where(tf.equal(masks, 0), paddings, outputs) # (h*N, T_q, T_k)
# Activation
outputs = tf.nn.softmax(outputs) # (h*N, T_q, T_k)
# Query Masking
query_masks = tf.sign(tf.abs(tf.reduce_sum(queries, axis=-1))) # (N, T_q)
query_masks = tf.tile(query_masks, [num_heads, 1]) # (h*N, T_q)
query_masks = tf.tile(tf.expand_dims(query_masks, -1), [1, 1, tf.shape(keys)[1]]) # (h*N, T_q, T_k)
outputs *= query_masks # broadcasting. (N, T_q, C)
# Dropouts
outputs = tf.layers.dropout(outputs, rate=dropout_rate, training=tf.convert_to_tensor(is_training))
# Weighted sum
outputs = tf.matmul(outputs, V_) # ( h*N, T_q, C/h)
# Restore shape
outputs = tf.concat(tf.split(outputs, num_heads, axis=0), axis=2) # (N, T_q, C)
# Residual connection
outputs += queries
# Normalize
outputs = normalize(outputs) # (N, T_q, C)
return outputs6）feedforward
前向网络
def feedforward(inputs,
num_units=[2048, 512],
scope="multihead_attention",
reuse=None):
with tf.variable_scope(scope, reuse=reuse):
# Inner layer
params = {"inputs": inputs, "filters": num_units[0], "kernel_size": 1,
"activation": tf.nn.relu, "use_bias": True}
outputs = tf.layers.conv1d(**params)
# Readout layer
params = {"inputs": outputs, "filters": num_units[1], "kernel_size": 1,
"activation": None, "use_bias": True}
outputs = tf.layers.conv1d(**params)
# Residual connection
outputs += inputs
# Normalize
outputs = normalize(outputs)
return outputs二 模型训练部分
模型图的构建如下所示：
class Graph():
def __init__(self, is_training=True):
self.graph = tf.Graph()
with self.graph.as_default():
if is_training:
self.x, self.y, self.num_batch = get_batch_data() # (N, T)
else: # inference
self.x = tf.placeholder(tf.int32, shape=(None, hp.maxlen))
self.y = tf.placeholder(tf.int32, shape=(None, hp.maxlen))
# define decoder inputs(??)
self.decoder_inputs = tf.concat((tf.ones_like(self.y[:, :1])*2, self.y[:, :-1]), -1) # 2: