[Misc] Add FA2 support to ViT MHA layer (#12355)

Signed-off-by: Isotr0py <2037008807@qq.com>

tests/kernels/test_mha_attn.py

+"""
+Test:
+
+* Tests for MultiHeadAttention layer
+"""
+from unittest.mock import patch
+
+import pytest
+import torch
+
+from vllm.attention.layer import MultiHeadAttention
+from vllm.attention.selector import _Backend, _cached_get_attn_backend
+from vllm.platforms import current_platform
+from vllm.platforms.cpu import CpuPlatform
+from vllm.platforms.cuda import CudaPlatform
+from vllm.platforms.rocm import RocmPlatform
+
+
+@pytest.fixture(autouse=True)
+def clear_cache():
+    """Clear lru cache to ensure each test case runs without caching.
+    """
+    _cached_get_attn_backend.cache_clear()
+
+
+@pytest.mark.parametrize("device", ["cpu", "hip", "cuda"])
+def test_mha_attn_platform(device: str):
+    """
+    Test that the attention selector between different platform and device.
+    """
+    torch.set_default_dtype(torch.float16)
+
+    if device == "cpu":
+        with patch("vllm.attention.selector.current_platform", CpuPlatform()):
+            attn = MultiHeadAttention(16, 64, scale=1)
+            assert attn.attn_backend == _Backend.TORCH_SDPA
+    elif device == "hip":
+        with patch("vllm.attention.selector.current_platform", RocmPlatform()):
+            attn = MultiHeadAttention(16, 64, scale=1)
+            assert attn.attn_backend == _Backend.TORCH_SDPA
+    else:
+        with patch("vllm.attention.selector.current_platform", CudaPlatform()):
+            attn = MultiHeadAttention(16, 64, scale=1)
+            assert attn.attn_backend == _Backend.FLASH_ATTN
+
+        with patch("vllm.attention.selector.current_platform", CudaPlatform()):
+            attn = MultiHeadAttention(16, 72, scale=1)
+            assert attn.attn_backend == _Backend.XFORMERS
+
+
+def ref_attention(
+    query: torch.Tensor,
+    key: torch.Tensor,
+    value: torch.Tensor,
+    scale: float,
+) -> torch.Tensor:
+    """
+    Native implementation of scaled dot product attention without mask:
+    - query, key, value: [batch_size, seq_len, num_heads, head_size]
+    - attn_mask: [batch_size, seq_len, seq_len]
+    """
+    query, key, value = (x.transpose(1, 2) for x in (query, key, value))
+    attn_weights = scale * torch.matmul(query, key.transpose(2, 3))
+    attn_weights = torch.softmax(attn_weights, dim=-1).to(value.dtype)
+    out = torch.matmul(attn_weights, value).transpose(1, 2)
+    return out
+
+
+BATCH_SIZES = [1, 16]
+SEQ_LENS = [1]
+NUM_HEADS = [1, 16]
+NUM_KV_HEADS = [1]
+HEAD_SIZES = [64, 80]
+# flshattF and tritonflashattF supported: {torch.float16, torch.bfloat16}
+DTYPES = [
+    torch.half, torch.bfloat16, torch.float
+] if not current_platform.is_rocm() else [torch.half, torch.bfloat16]
+CUDA_DEVICES = ["cuda"]
+
+
+@pytest.mark.parametrize("batch_size", BATCH_SIZES)
+@pytest.mark.parametrize("seq_len", SEQ_LENS)
+@pytest.mark.parametrize("num_heads", NUM_HEADS)
+@pytest.mark.parametrize("num_kv_heads", NUM_KV_HEADS)
+@pytest.mark.parametrize("head_size", HEAD_SIZES)
+@pytest.mark.parametrize("dtype", DTYPES)
+@pytest.mark.parametrize("device", CUDA_DEVICES)
+def test_mha_attn_forward(
+    batch_size: int,
+    seq_len: int,
+    num_heads: int,
+    num_kv_heads: int,
+    head_size: int,
+    dtype: torch.dtype,
+    device: str,
+):
+    current_platform.seed_everything(0)
+    torch.set_default_device(device)
+    torch.set_default_dtype(dtype)
+
+    q = torch.randn(batch_size, seq_len, num_heads * head_size)
+    k = torch.randn(batch_size, seq_len, num_kv_heads * head_size)
+    v = torch.randn(batch_size, seq_len, num_kv_heads * head_size)
+    scale = 1.0 / head_size**0.5
+    attn = MultiHeadAttention(num_heads,
+                              head_size,
+                              scale=scale,
+                              num_kv_heads=num_kv_heads)
+    output = attn(q, k, v)
+
+    assert num_heads % num_kv_heads == 0
+    num_queries_per_kv = num_heads // num_kv_heads
+    q = q.reshape(batch_size, seq_len, num_heads, head_size)
+    k = k.reshape(batch_size, seq_len, num_kv_heads, head_size)
+    v = v.reshape(batch_size, seq_len, num_kv_heads, head_size)
+    if num_queries_per_kv > 1:
+        k = torch.repeat_interleave(k, num_queries_per_kv, dim=2)
+        v = torch.repeat_interleave(v, num_queries_per_kv, dim=2)
+
+    ref_output = ref_attention(
+        q,
+        k,
+        v,
+        scale=scale,
+    ).reshape(batch_size, seq_len, num_heads * head_size)
+    torch.testing.assert_close(output, ref_output)

vllm/attention/layer.py

@@ -210,6 +210,9 @@ class MultiHeadAttention(nn.Module):
         self.scale = scale
         self.num_kv_heads = num_heads if num_kv_heads is None else num_kv_heads
 
+        assert self.num_heads % self.num_kv_heads == 0
+        self.num_queries_per_kv = self.num_heads // self.num_kv_heads
+
         dtype = torch.get_default_dtype()
         attn_backend = get_attn_backend(head_size,
                                         dtype,
@@ -217,11 +220,12 @@ class MultiHeadAttention(nn.Module):
                                         block_size=16,
                                         is_attention_free=False)
         backend = backend_name_to_enum(attn_backend.get_name())
-        if backend in {_Backend.FLASH_ATTN, _Backend.FLASH_ATTN_VLLM_V1}:
-            backend = _Backend.XFORMERS
 
         self.attn_backend = backend if backend in {
-            _Backend.TORCH_SDPA, _Backend.XFORMERS
+            _Backend.TORCH_SDPA,
+            _Backend.XFORMERS,
+            _Backend.FLASH_ATTN,
+            _Backend.FLASH_ATTN_VLLM_V1,
         } else _Backend.TORCH_SDPA
 
     def forward(
@@ -231,7 +235,6 @@ class MultiHeadAttention(nn.Module):
         value: torch.Tensor,
     ) -> torch.Tensor:
         """Input shape: batch_size x seq_len x hidden_size"""
-        # TODO(Isotr0py): Use existing backend implementations and support FA2
         bsz, q_len, _ = query.size()
         kv_len = key.size(1)
 
@@ -239,7 +242,19 @@ class MultiHeadAttention(nn.Module):
         key = key.view(bsz, kv_len, self.num_kv_heads, self.head_size)
         value = value.view(bsz, kv_len, self.num_kv_heads, self.head_size)
 
-        if self.attn_backend == _Backend.XFORMERS:
+        if (num_repeat := self.num_queries_per_kv) > 1:
+            # Handle MQA and GQA
+            key = torch.repeat_interleave(key, num_repeat, dim=2)
+            value = torch.repeat_interleave(value, num_repeat, dim=2)
+
+        if self.attn_backend in {
+                _Backend.FLASH_ATTN,
+                _Backend.FLASH_ATTN_VLLM_V1,
+        }:
+            from vllm.vllm_flash_attn import flash_attn_func
+
+            out = flash_attn_func(query, key, value, softmax_scale=self.scale)
+        elif self.attn_backend == _Backend.XFORMERS:
             from xformers import ops as xops
 
             out = xops.memory_efficient_attention_forward(query,