Support flash attn 2 (#72)

Author: jlamypoirier

megatron/model/transformer.py

@@ -6,9 +6,8 @@
 import numpy as np
 import torch
 import torch.nn.functional as F
-
 from typing import Optional
-from torch import nn
+from packaging.version import Version
 
 from megatron import get_timers, get_args, get_retro_args, core, get_num_microbatches
 from megatron.utils import print_rank_0
@@ -36,9 +35,15 @@
     rearrange = None
 
 try:
-    from flash_attn.flash_attn_interface import flash_attn_unpadded_func
+    import flash_attn as _flash_attn
+    if Version(getattr(_flash_attn, "__version__", "1")) >= Version("2"):
+        from flash_attn.flash_attn_interface import flash_attn_func
+        FLASH_VERSION = 2
+    else:
+        from flash_attn.flash_attn_interface import flash_attn_unpadded_func
+        FLASH_VERSION = 1
 except ImportError:
-    flash_attn_unpadded_func = None
+    FLASH_VERSION = None
 
 
 """ We use the following notation throughout this file:
@@ -508,7 +513,7 @@ class FlashSelfAttention(torch.nn.Module):
     def __init__(self, causal=False, softmax_scale=None, attention_dropout=0.0,
                  device=None, dtype=None):
         super().__init__()
-        assert flash_attn_unpadded_func is not None, ('Please install FlashAttention first, '
+        assert FLASH_VERSION is not None, ('Please install FlashAttention first, '
                                                       'e.g., with pip install flash-attn')
         assert rearrange is not None, 'Please install einops first, e.g., with pip install einops'
         self.causal = causal
@@ -521,10 +526,31 @@ def forward(self, q, k, v):
         ---------
             q, k, v: The tensor containing the query, key, and value. (B, S, H, D)
         """
-
         assert all((i.dtype in [torch.float16, torch.bfloat16] for i in (q,k,v)))
         assert all((i.is_cuda for i in (q,k,v)))
 
+        if FLASH_VERSION==1:
+            return self._forward_v1(q,k,v)
+
+        seqlen_q, seqlen_k = q.shape[1], k.shape[1]
+
+        if self.training:
+            # during training q,k,v always have same seqlen
+            assert seqlen_k == seqlen_q
+            is_causal = self.causal
+            dropout_p = self.dropout_p
+        else:
+            # turn off FA causal mask after first inference autoregressive iteration
+            # only on first autoregressive step q,k,v have same seqlen
+            is_causal = self.causal and (seqlen_q == seqlen_k)
+            dropout_p = 0
+
+        output = flash_attn_func(q, k, v, dropout_p,softmax_scale=self.softmax_scale, causal=is_causal)
+
+        return output
+
+
+    def _forward_v1(self, q, k, v):
         batch_size, seqlen_q = q.shape[0], q.shape[1]
         seqlen_k = k.shape[1]
 
@@ -647,7 +673,7 @@ def __init__(self, init_method,
         self.checkpoint_core_attention = args.recompute_granularity == 'selective'
 
         if self.use_flash_attn:
-            if flash_attn_unpadded_func is None:
+            if FLASH_VERSION is None:
                 raise ImportError('FlashAttention is not installed, please install with '
                                   'pip install flash-attn')
             assert attention_type == AttnType.self_attn, ('FlashAttention code path only supports '
@@ -882,6 +908,7 @@ def forward(self, hidden_states, attention_mask,
                 sq, b, np, hn = query_layer.size()
                 # Expand kv to be compatible with flash-attn implementation
                 # [sq, b, 1, hn] -> [sq, b, np, hn]
+                # TODO: This should be skippable for flash 2, but getting illegal memory access.
                 key_layer = key_layer.expand((sq, b, np, hn))
                 value_layer = value_layer.expand((sq, b, np, hn))
             q, k, v = [rearrange(x, 's b ... -> b s ...').contiguous()