[Performance] Extract KV cache update op from flashinfer forward (#35422)

Signed-off-by: ElizaWszola <ewszola@redhat.com>

vllm/v1/attention/backends/flashinfer.py

@@ -381,6 +381,8 @@ class FlashInferBackend(AttentionBackend):
             return "HND"
         return None
 
+    forward_includes_kv_cache_update: bool = False
+
 
 @dataclass
 class FIPrefill:
@@ -1330,28 +1332,11 @@ class FlashInferImpl(AttentionImpl):
 
         num_actual_tokens = attn_metadata.num_actual_tokens
 
-        if self.kv_sharing_target_layer_name is None:
-            # Reshape the input keys and values and store them in the cache.
-            # Skip this if sharing KV cache with an earlier attention layer.
-            # NOTE(woosuk): Here, key and value are padded while slot_mapping is
-            # not padded. However, we don't need to do key[:num_actual_tokens]
-            # and value[:num_actual_tokens] because the reshape_and_cache_flash
-            # op uses the slot_mapping's shape to determine the number of
-            # actual tokens.
-            torch.ops._C_cache_ops.reshape_and_cache_flash(
-                key,
-                value,
-                kv_cache[:, 0],
-                kv_cache[:, 1],
-                attn_metadata.slot_mapping,
-                self.kv_cache_dtype,
-                layer._k_scale,
-                layer._v_scale,
-            )
-
         # The FlashInfer api requires data to be in fp8_e4m3 or fp8_e5m2
         # to process the cache when the kv_cache_dtype is fp8
-            if self.kv_cache_dtype.startswith("fp8"):
+        if self.kv_sharing_target_layer_name is None and self.kv_cache_dtype.startswith(
+            "fp8"
+        ):
             torch_dtype = FlashInferBackend.get_fp8_dtype_for_flashinfer(
                 self.kv_cache_dtype
             )
@@ -1599,6 +1584,33 @@ class FlashInferImpl(AttentionImpl):
                 )
         return output_padded
 
+    def do_kv_cache_update(
+        self,
+        layer: torch.nn.Module,
+        key: torch.Tensor,
+        value: torch.Tensor,
+        kv_cache: torch.Tensor,
+        slot_mapping: torch.Tensor,
+    ) -> None:
+        if self.kv_sharing_target_layer_name is None:
+            # Reshape the input keys and values and store them in the cache.
+            # Skip this if sharing KV cache with an earlier attention layer.
+            # NOTE(woosuk): Here, key and value are padded while slot_mapping is
+            # not padded. However, we don't need to do key[:num_actual_tokens]
+            # and value[:num_actual_tokens] because the reshape_and_cache_flash
+            # op uses the slot_mapping's shape to determine the number of
+            # actual tokens.
+            torch.ops._C_cache_ops.reshape_and_cache_flash(
+                key,
+                value,
+                kv_cache[:, 0],
+                kv_cache[:, 1],
+                slot_mapping,
+                self.kv_cache_dtype,
+                layer._k_scale,
+                layer._v_scale,
+            )
+
 
 def fast_plan_decode(
     self,  # decode wrapper