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attn_mask_utils.py
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from typing import List, Optional, Tuple, Union
import torch
from transformers.modeling_attn_mask_utils import AttentionMaskConverter
def _prepare_4d_causal_attention_mask(
attention_mask: Optional[torch.Tensor],
input_shape: Union[torch.Size, Tuple, List],
inputs_embeds: torch.Tensor,
past_key_values_length: int,
sliding_window: Optional[int] = None,
):
"""
Creates a causal 4D mask of shape `(batch_size, 1, query_length, key_value_length)` from a 2D mask of shape
`(batch_size, key_value_length)`
Args:
attention_mask (`torch.Tensor` or `None`):
A 2D attention mask of shape `(batch_size, key_value_length)`
input_shape (`tuple(int)` or `list(int)` or `torch.Size`):
The input shape should be a tuple that defines `(batch_size, query_length)`.
inputs_embeds (`torch.Tensor`):
The embedded inputs as a torch Tensor.
past_key_values_length (`int`):
The length of the key value cache.
sliding_window (`int`, *optional*):
If the model uses windowed attention, a sliding window should be passed.
"""
attn_mask_converter = AttentionMaskConverter(
is_causal=False, sliding_window=sliding_window
) # is_causal=True in original implementation
key_value_length = input_shape[-1] + past_key_values_length
# 4d mask is passed through the layers
if attention_mask is not None and len(attention_mask.shape) == 2:
attention_mask = attn_mask_converter.to_4d(
attention_mask,
input_shape[-1],
key_value_length=key_value_length,
dtype=inputs_embeds.dtype,
)
elif attention_mask is not None and len(attention_mask.shape) == 4:
expected_shape = (input_shape[0], 1, input_shape[1], key_value_length)
if tuple(attention_mask.shape) != expected_shape:
raise ValueError(
f"Incorrect 4D attention_mask shape: {tuple(attention_mask.shape)}; expected: {expected_shape}."
)
else:
# if the 4D mask has correct shape - invert it and fill with negative infinity
inverted_mask = 1.0 - attention_mask
attention_mask = inverted_mask.masked_fill(
inverted_mask.to(torch.bool), torch.finfo(inputs_embeds.dtype).min
)
else:
attention_mask = attn_mask_converter.to_causal_4d(
input_shape[0],
input_shape[-1],
key_value_length,
dtype=inputs_embeds.dtype,
device=inputs_embeds.device,
)
return attention_mask
# Adapted from _prepare_4d_causal_attention_mask
def _prepare_4d_causal_attention_mask_for_sdpa(
attention_mask: Optional[torch.Tensor],
input_shape: Union[torch.Size, Tuple, List],
inputs_embeds: torch.Tensor,
past_key_values_length: int,
sliding_window: Optional[int] = None,
):
"""
Prepares the correct `attn_mask` argument to be used by `torch.nn.functional.scaled_dot_product_attention`.
In case no token is masked in the `attention_mask` argument, we simply set it to `None` for the cases `query_length == 1` and
`key_value_length == query_length`, and rely instead on SDPA `is_causal` argument to use causal/non-causal masks,
allowing to dispatch to the flash attention kernel (that can otherwise not be used if a custom `attn_mask` is passed).
"""
attn_mask_converter = AttentionMaskConverter(
is_causal=False, sliding_window=sliding_window
) # is_causal=True in original implementation
key_value_length = input_shape[-1] + past_key_values_length
batch_size, query_length = input_shape
# torch.jit.trace, symbolic_trace and torchdynamo with fullgraph=True are unable to capture the controlflow `is_causal=attention_mask is None and q_len > 1`
# used as an SDPA argument. We keep compatibility with these tracing tools by always using SDPA's `attn_mask` argument in case we are tracing.
# TODO: For dynamo, rather use a check on fullgraph=True once this is possible (https://github.com/pytorch/pytorch/pull/120400).
is_tracing = (
torch.jit.is_tracing()
or isinstance(inputs_embeds, torch.fx.Proxy)
or (hasattr(torch, "_dynamo") and torch._dynamo.is_compiling())
)
if attention_mask is not None:
# 4d mask is passed through
if len(attention_mask.shape) == 4:
expected_shape = (input_shape[0], 1, input_shape[1], key_value_length)
if tuple(attention_mask.shape) != expected_shape:
raise ValueError(
f"Incorrect 4D attention_mask shape: {tuple(attention_mask.shape)}; expected: {expected_shape}."
)
else:
# if the 4D mask has correct shape - invert it and fill with negative infinity
inverted_mask = 1.0 - attention_mask.to(inputs_embeds.dtype)
attention_mask = inverted_mask.masked_fill(
inverted_mask.to(torch.bool), torch.finfo(inputs_embeds.dtype).min
)
return attention_mask
elif not is_tracing and torch.all(attention_mask == 1):
if query_length == 1:
# For query_length == 1, causal attention and bi-directional attention are the same.
attention_mask = None
elif key_value_length == query_length:
attention_mask = None
else:
# Unfortunately, for query_length > 1 and key_value_length != query_length, we cannot generally ignore the attention mask, as SDPA causal mask generation
# may be wrong. We will set `is_causal=False` in SDPA and rely on Transformers attention_mask instead, hence not setting it to None here.
# Reference: https://github.com/pytorch/pytorch/issues/108108
pass
elif query_length > 1 and key_value_length != query_length:
# See the comment above (https://github.com/pytorch/pytorch/issues/108108).
# Ugly: we set it to True here to dispatch in the following controlflow to `to_causal_4d`.
attention_mask = True
elif is_tracing:
raise ValueError(
'Attention using SDPA can not be traced with torch.jit.trace when no attention_mask is provided. To solve this issue, please either load your model with the argument `attn_implementation="eager"` or pass an attention_mask input when tracing the model.'
)
if attention_mask is None:
expanded_4d_mask = None
elif attention_mask is True:
expanded_4d_mask = attn_mask_converter.to_causal_4d(
input_shape[0],
input_shape[-1],
key_value_length,
dtype=inputs_embeds.dtype,
device=inputs_embeds.device,
)
else:
expanded_4d_mask = attn_mask_converter.to_4d(
attention_mask,
input_shape[-1],
dtype=inputs_embeds.dtype,
key_value_length=key_value_length,
)
# Attend to all tokens in masked rows from the causal_mask, for example the relevant first rows when
# using left padding. This is required by F.scaled_dot_product_attention memory-efficient attention path.
# Details: https://github.com/pytorch/pytorch/issues/110213
if not is_tracing and expanded_4d_mask.device.type == "cuda":
expanded_4d_mask = AttentionMaskConverter._unmask_unattended(
expanded_4d_mask, min_dtype=torch.finfo(inputs_embeds.dtype).min
)
return expanded_4d_mask