[ROCm][AMD][Model]Adding alibi slopes support in ROCm triton flash at…

…tention and naive flash attention (vllm-project#6043)

Co-authored-by: Hongxia Yang <[email protected]>

vllm/attention/backends/rocm_flash_attn.py

@@ -166,6 +166,37 @@ def decode_metadata(self) -> Optional["ROCmFlashAttentionMetadata"]:
         return self._cached_decode_metadata
 
 
+def _make_alibi_bias(alibi_slopes: torch.Tensor,
+                     dtype: torch.dtype,
+                     seq_lens: Optional[List[int]],
+                     make_attn_mask: bool = True) -> List[torch.Tensor]:
+    attn_biases = []
+    if seq_lens:
+        for seq_len in seq_lens:
+            bias = torch.arange(seq_len, dtype=dtype)
+            # NOTE(zhuohan): HF uses
+            #     `bias = bias[None, :].repeat(seq_len, 1)`
+            # here. We find that both biases give the same results, but
+            # the bias below more accurately follows the original ALiBi
+            # paper.
+            bias = bias[None, :] - bias[:, None]
+
+            num_heads = alibi_slopes.shape[0]
+            bias = bias[None, :].repeat(
+                (num_heads, 1, 1)).to(alibi_slopes.device)
+            bias.mul_(alibi_slopes[:, None, None])
+            if make_attn_mask:
+                inf_mask = torch.empty(
+                    (1, seq_len, seq_len),
+                    dtype=bias.dtype).fill_(-torch.inf).triu_(diagonal=1).to(
+                        alibi_slopes.device)
+                attn_biases.append((bias + inf_mask).to(dtype))
+            else:
+                attn_biases.append(bias.to(dtype))
+
+    return attn_biases
+
+
 class ROCmFlashAttentionImpl(AttentionImpl):
     """
     If the input tensors contain prompt tokens, the layout is as follows:
@@ -324,7 +355,14 @@ def forward(
                 # triton attention
                 # When block_tables are not filled, it means q and k are the
                 # prompt, and they have the same length.
+                attn_masks = None
                 if self.use_triton_flash_attn:
+                    if self.alibi_slopes is not None:
+                        attn_masks = _make_alibi_bias(
+                            self.alibi_slopes,
+                            query.dtype,
+                            attn_metadata.seq_lens,
+                            make_attn_mask=False)  # type: ignore
                     out, _ = self.attn_func(
                         query,
                         key,
@@ -336,12 +374,20 @@ def forward(
                         prefill_meta.max_prefill_seq_len,
                         True,
                         self.scale,
+                        attn_masks[0][None]
+                        if attn_masks is not None else None,
                     )
                 elif self.use_naive_attn:
                     if self.num_kv_heads != self.num_heads:
                         # Interleave for MQA workaround.
                         key = self.repeat_kv(key, self.num_queries_per_kv)
                         value = self.repeat_kv(value, self.num_queries_per_kv)
+                    if self.alibi_slopes is not None:
+                        attn_masks = _make_alibi_bias(
+                            self.alibi_slopes,
+                            query.dtype,
+                            attn_metadata.seq_lens,
+                            make_attn_mask=True)  # type: ignore
                     query = query.movedim(0, query.dim() - 2)
                     key = key.movedim(0, key.dim() - 2)
                     value = value.movedim(0, value.dim() - 2)
@@ -355,6 +401,7 @@ def forward(
                         self.num_heads,
                         self.head_size,
                         self.scale,
+                        attn_masks,
                     )
                 else:
                     out = self.attn_func(
@@ -418,13 +465,14 @@ def _sdpa_attention(
     num_heads: int,
     head_size: int,
     scale: float,
+    attn_masks: Optional[List[torch.Tensor]] = None,
 ) -> torch.Tensor:
     start = 0
     output = torch.empty((num_tokens, num_heads, head_size),
                          dtype=query.dtype,
                          device=query.device)
 
-    for seq_len in seq_lens:
+    for i, seq_len in enumerate(seq_lens):
         end = start + seq_len
         with torch.backends.cuda.sdp_kernel(enable_math=True,
                                             enable_flash=False,
@@ -434,7 +482,8 @@ def _sdpa_attention(
                 key[:, start:end, :],
                 value[:, start:end, :],
                 dropout_p=0.0,
-                is_causal=True,
+                is_causal=attn_masks is None,
+                attn_mask=attn_masks[i] if attn_masks else None,
                 scale=scale).movedim(query.dim() - 2, 0)
             output[start:end, :, :] = sub_out
             start = end