[BugFix] fix: aot passes kvcache dtype information (#19750)

Signed-off-by: Mickael Seznec <mickael@mistral.ai>

vllm/v1/attention/backends/flash_attn.py

@@ -99,6 +99,13 @@ class FlashAttentionBackend(AttentionBackend):
             raise ValueError(f"Unknown cache layout format {cache_layout}.")
         return stride_order
 
+    @staticmethod
+    def get_fp8_dtype_for_flashattn(kv_cache_dtype: str) -> torch.dtype:
+        if kv_cache_dtype in ("fp8", "fp8_e4m3"):
+            return torch.float8_e4m3fn
+        else:
+            raise ValueError(f"Unrecognized FP8 dtype: {kv_cache_dtype}")
+
 
 @dataclass
 class FlashAttentionMetadata:
@@ -161,6 +168,7 @@ class FlashAttentionMetadataBuilder(
             self.parallel_config)
         self.num_heads_kv = self.model_config.get_num_kv_heads(
             self.parallel_config)
+        self.kv_cache_dtype = kv_cache_spec.dtype
         self.headdim = self.model_config.get_head_size()
         self.block_size = kv_cache_spec.block_size
 
@@ -239,17 +247,24 @@ class FlashAttentionMetadataBuilder(
 
         def schedule(batch_size, cu_query_lens, max_query_len, seqlens,
                      max_seq_len, causal):
+            cache_dtype = self.cache_config.cache_dtype
+            if cache_dtype.startswith("fp8"):
+                qkv_dtype = FlashAttentionBackend.get_fp8_dtype_for_flashattn(
+                    cache_dtype)
+            else:
+                qkv_dtype = self.kv_cache_dtype
             if aot_schedule:
                 return get_scheduler_metadata(
                     batch_size=batch_size,
                     max_seqlen_q=max_query_len,
                     max_seqlen_k=max_seq_len,
-                    cache_seqlens=seqlens,
                     num_heads_q=self.num_heads_q,
                     num_heads_kv=self.num_heads_kv,
                     headdim=self.headdim,
-                    page_size=self.block_size,
+                    cache_seqlens=seqlens,
+                    qkv_dtype=qkv_dtype,
                     cu_seqlens_q=cu_query_lens,
+                    page_size=self.block_size,
                     causal=causal,
                     window_size=self.aot_sliding_window,
                     num_splits=self.max_num_splits,
@@ -474,8 +489,10 @@ class FlashAttentionImpl(AttentionImpl):
             )
 
         if self.kv_cache_dtype.startswith("fp8"):
-            key_cache = key_cache.view(torch.float8_e4m3fn)
-            value_cache = value_cache.view(torch.float8_e4m3fn)
+            dtype = FlashAttentionBackend.get_fp8_dtype_for_flashattn(
+                self.kv_cache_dtype)
+            key_cache = key_cache.view(dtype)
+            value_cache = value_cache.view(dtype)
             num_tokens, num_heads, head_size = query.shape
             query, _ = ops.scaled_fp8_quant(
                 query.reshape(