fix multi-step + rocm_flash_attn support

vllm/attention/backends/rocm_flash_attn.py

@@ -191,12 +191,22 @@ def decode_metadata(self) -> Optional["ROCmFlashAttentionMetadata"]:
         )
         return self._cached_decode_metadata
 
-    def advance_step(self, model_input: "ModelInputForGPUWithSamplingMetadata",
+    def advance_step(self,
+                     model_input: "ModelInputForGPUWithSamplingMetadata",
                      sampled_token_ids: Optional[torch.Tensor],
-                     block_size: int, num_seqs: int, num_queries: int):
+                     block_size: int,
+                     num_seqs: int,
+                     num_queries: int,
+                     turn_prefills_into_decodes: bool = False):
         """
         Update metadata in-place to advance one decode step.
         """
+
+        assert not turn_prefills_into_decodes, \
+            ("Chunked prefill is not supported with rocm_flash_attn yet."
+             "turn_prefills_into_decodes is a Multi-Step + Chunked-Prefill "
+             "specific parameter.")
+
         # When using cudagraph, the num_seqs is padded to the next captured
         # batch sized, but num_queries tracks the actual number of requests in
         # the batch. For --enforce-eager mode, num_seqs == num_queries