[BugFix] Address PrefillCacheHit state to fix prefix cache accuracy bug (#1498)

When use AscendScheduler with prefix-cache enabled and chunk-prefill
disabled, there will be accuray problem because there is no branch in
mla_v1 to process this scenario. This PR fixes it.

Signed-off-by: whx-sjtu <2952154980@qq.com>

Author: whx-sjtu

vllm_ascend/attention/mla_v1.py

@@ -758,7 +758,8 @@ def _forward_prefill(
 
         if attn_metadata.attn_state in [
                 AscendAttentionState.ChunkedPrefill,
-                AscendAttentionState.SpecDecoding
+                AscendAttentionState.SpecDecoding,
+                AscendAttentionState.PrefillCacheHit
         ] and not ascend_config.chunked_prefill_for_mla:
             attn_output_torch = torch.empty(num_tokens,
                                             self.num_heads * self.v_head_dim,
@@ -783,7 +784,8 @@ def _forward_prefill(
                 causal=True)
         elif attn_metadata.attn_state in [
                 AscendAttentionState.ChunkedPrefill,
-                AscendAttentionState.SpecDecoding
+                AscendAttentionState.SpecDecoding,
+                AscendAttentionState.PrefillCacheHit
         ]:
             attn_lse = torch.empty(self.num_heads,
                                    num_tokens,
@@ -835,13 +837,14 @@ def _forward_prefill(
             attn_output = attn_output.view(-1, self.num_heads, self.v_head_dim)
         else:
             raise RuntimeError(
-                "Unexpected path reached, AscendMLAImpl should only have PrefillNoCache, ChunkedPrefill and SpecDecoding scenario in forward prefill, please file a bug to vllm-ascend !"
+                "Unexpected path reached, AscendMLAImpl should only have PrefillNoCache, PrefillCacheHit, ChunkedPrefill and SpecDecoding scenario in forward prefill, please file a bug to vllm-ascend !"
             )
         attn_output = attn_output.reshape(
             [num_tokens, self.num_heads * self.v_head_dim])
         if attn_metadata.attn_state in [
                 AscendAttentionState.ChunkedPrefill,
-                AscendAttentionState.SpecDecoding
+                AscendAttentionState.SpecDecoding,
+                AscendAttentionState.PrefillCacheHit
         ] and not ascend_config.chunked_prefill_for_mla:
             attn_output = attn_output_torch