Prefix-Tuning源码解析
Prefix-Tuning在PEFT包中的源码实现
改写自Based on https://github.com/THUDM/P-tuning-v2/blob/main/model/prefix_encoder.py
python
import torch
from transformers import PretrainedConfig
class PrefixEncoder(torch.nn.Module):
r'''
The torch.nn model to encode the prefix
Input shape: (batch-size, prefix-length)
Output shape: (batch-size, prefix-length, 2*layers*hidden)
'''
def __init__(self, config):
super().__init__()
self.prefix_projection = config.prefix_projection
if self.prefix_projection:
# Use a two-layer MLP to encode the prefix
self.embedding = torch.nn.Embedding(config.prefix_length, config.hidden_size)
self.trans = torch.nn.Sequential(
torch.nn.Linear(config.hidden_size, config.encoder_hidden_size),
torch.nn.Tanh(),
torch.nn.Linear(config.encoder_hidden_size, config.num_hidden_layers * 2 * config.hidden_size)
)
else:
self.embedding = torch.nn.Embedding(config.prefix_length, config.num_hidden_layers * 2 * config.hidden_size)
def forward(self, prefix: torch.Tensor):
if self.prefix_projection:
prefix_tokens = self.embedding(prefix)
past_key_values = self.trans(prefix_tokens)
else:
past_key_values = self.embedding(prefix)
return past_key_values
if __name__ == "__main__":
configs = {"prefix_length":20,
"hidden_size":768,
"encoder_hidden_size":768,
"num_hidden_layers":12,
"prefix_projection":False
}
prefix_encoder = PrefixEncoder(config=PretrainedConfig.from_dict(configs))
print(prefix_encoder)
batch_size = 8
prefix = torch.arange(20).long().expand(batch_size, -1)
print(prefix.shape)
output = prefix_encoder(prefix)
print(output.shape)下面我们以T5-large模型为例子：
不考虑Use a two-layer MLP to encode the prefix的话，prefix tuning主要包括以下代码：
python
class PrefixEncoder(torch.nn.Module):
def __init__(self, config):
super().__init__()
...
self.embedding = torch.nn.Embedding(num_virtual_tokens, num_layers * 2 * token_dim) #num_virtual_tokens=20,token_dim=1024,num_layers=24
def forward(self, prefix: torch.Tensor):
past_key_values = self.embedding(prefix)
return past_key_values得到的PrefixEncoder被传入peft->peft_model.py->prompt_encoder：
python
PrefixEncoder(
(embedding): Embedding(20, 49152) # 1024*2*24
)self.prompt_tokens初始化为长度2*20的向量，因为T5有编码器和解码器，需要两次prefix：
python
self.prompt_tokens[adapter_name] = torch.arange(
config.num_virtual_tokens * config.num_transformer_submodules
).long() #20*2
# tensor([ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,
# 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35,
# 36, 37, 38, 39])
python
prompt_tokens = (
self.prompt_tokens[self.active_adapter]
.unsqueeze(0)
.expand(batch_size, -1)
.to(prompt_encoder.embedding.weight.device)
)
prompt_tokens = prompt_tokens[:, : peft_config.num_virtual_tokens]
# 此时prompt_tokens.shape = (batch_size=8, num_virtual_tokens=20)
past_key_values = prompt_encoder(prompt_tokens)
torch.Size([8, 20, 49152])但目前的past_key_values还是所有层的集合，我们需要把past_key_values分解为每一层：
python
past_key_values = past_key_values.view(
batch_size, #8
peft_config.num_virtual_tokens, #20
peft_config.num_layers * 2, #24*2
peft_config.num_attention_heads, #16
peft_config.token_dim // peft_config.num_attention_heads, #1024/16
)
# torch.Size([8, 20, 48, 16, 64])因为有编码器和解码器，所以再复制一次
python
past_key_values = torch.cat([past_key_values, past_key_values], dim=2)
# torch.Size([8, 20, 96, 16, 64])
# 重排：torch.Size([96, 8, 16, 20, 64])
# 然后split成一个长度为24的tuple，每个tuple的shape：torch.Size([4, 8, 16, 20, 64])
past_key_values = past_key_values.permute([2, 0, 3, 1, 4]).split(
peft_config.num_transformer_submodules * 2
)也就是说past_key_values是24个层的Prefix embedding，形状为`(num_transformer_submodules * 2, batch_size, num_attention_heads, num_virtual_tokens, token_dim/num_attention_heads])
注意这里*2是因为key+value.
transformers->models->t5->modeling_t5.py->T5Attention类，这里的关键步骤是project函数中的hidden_states = torch.cat([past_key_value, hidden_states], dim=2)，注意project函数仅仅用于key和value。
python
def forward(
self,
hidden_states,
mask=None,
key_value_states=None,
position_bias=None,
past_key_value=None,
layer_head_mask=None,
query_length=None,
use_cache=False,
output_attentions=False,
):
"""
Self-attention (if key_value_states is None) or attention over source sentence (provided by key_value_states).
"""
# Input is (batch_size, seq_length, dim)
# Mask is (batch_size, key_length) (non-causal) or (batch_size, key_length, key_length)
# past_key_value[0] is (batch_size, n_heads, q_len - 1, dim_per_head)
batch_size, seq_length = hidden_states.shape[:2]
real_seq_length = seq_length
if past_key_value is not None:
if len(past_key_value) != 2:
raise ValueError(
f"past_key_value should have 2 past states: keys and values. Got { len(past_key_value)} past states"
)
real_seq_length += past_key_value[0].shape[2] if query_length is None else query_length
key_length = real_seq_length if key_value_states is None else key_value_states.shape[1]
def shape(states):
"""projection"""
return states.view(batch_size, -1, self.n_heads, self.key_value_proj_dim).transpose(1, 2)
def unshape(states):
"""reshape"""
return states.transpose(1, 2).contiguous().view(batch_size, -1, self.inner_dim)
def project(hidden_states, proj_layer, key_value_states, past_key_value):
"""projects hidden states correctly to key/query states"""
if key_value_states is None:
# self-attn
# (batch_size, n_heads, seq_length, dim_per_head)
hidden_states = shape(proj_layer(hidden_states))
elif past_key_value is None:
# cross-attn
# (batch_size, n_heads, seq_length, dim_per_head)
hidden_states = shape(proj_layer(key_value_states))
if past_key_value is not None:
if key_value_states is None:
# self-attn
# (batch_size, n_heads, key_length, dim_per_head)
# 注意这里是重点：用串联方式
hidden_states = torch.cat([past_key_value, hidden_states], dim=2)
elif past_key_value.shape[2] != key_value_states.shape[1]:
# checking that the ` sequence_length` of the `past_key_value` is the same as
# the provided `key_value_states` to support prefix tuning
# cross-attn
# (batch_size, n_heads, seq_length, dim_per_head)
hidden_states = shape(proj_layer(key_value_states))
else:
# cross-attn
hidden_states = past_key_value
return hidden_states
real_seq_length += past_key_value[0].shape[2] if query_length is None else query_length分别计算query_states、key_states、value_states，用query和key计算attention score，得到score形状为torch.Size([8, 16, 2, 22])，所以输入X可以attend to itself以及prefix。
python
# hidden_states shape: torch.Size([8, 2, 1024])
# get query states
query_states = shape(self.q(hidden_states)) # (batch_size, n_heads, seq_length, dim_per_head)
# query_states shape: torch.Size([8, 16, 2, 64])
# get key/value states
key_states = project(
hidden_states, self.k, key_value_states, past_key_value[0] if past_key_value is not None else None
)
# key_states shape: torch.Size([8, 16, 22, 64])
value_states = project(
hidden_states, self.v, key_value_states, past_key_value[1] if past_key_value is not None else None
)
# value_states shape: torch.Size([8, 16, 22, 64])
# compute scores
# torch.Size([8, 16, 2, 22])
scores = torch.matmul(
query_states, key_states.transpose(3, 2)
) # equivalent of torch.einsum("bnqd,bnkd->bnqk", query_states, key_states), compatible with onnx op>9接下来就是经典的attention操作了。用attn_weights ([8, 16, 2, 22]) 和value_states ([8, 16, 22, 64])相乘，把22消掉，就是每个输入X的输出了。
python
# if key and values are already calculated
# we want only the last query position bias
# position_bias.shape: torch.Size([8, 16, 2, 22])
scores += position_bias_masked
attn_weights = nn.functional.softmax(scores.float(), dim=-1).type_as(
scores
) # (batch_size, n_heads, seq_length, key_length)
attn_weights = nn.functional.dropout(
attn_weights, p=self.dropout, training=self.training
) # (batch_size, n_heads, seq_length, key_length)
attn_output = unshape(torch.matmul(attn_weights, value_states)) # (batch_size, seq_length, dim) torch.Size([8, 2, 1024])
attn_output = self.o(attn_output)
present_key_value_state = (key_states, value_states) if (self.is_decoder and use_cache) else None
outputs = (attn_output,) + (present_key_value_state,) + (position_bias,)
if output_attentions:
outputs = outputs + (attn_weights,)
return outputs参考
https://huggingface.co/docs/peft/task_guides/seq2seq-prefix-tuning