fix(ROCm): Make flash_attn import optional in MLA attention (#33511)

Signed-off-by: rabi <ramishra@redhat.com>

vllm/model_executor/layers/attention/mla_attention.py

@@ -919,10 +919,20 @@ try:
 
     is_vllm_fa = True
 except ImportError:
-    # For rocm use upstream flash attention
+    is_vllm_fa = False
+    flash_attn_varlen_func = None  # type: ignore[assignment]
+    # On ROCm, vllm_flash_attn is not available, try upstream flash_attn instead.
+    # On CUDA, vllm_flash_attn should always be available (built with vLLM),
+    # so we don't attempt the fallback there.
     if current_platform.is_rocm():
+        try:
             from flash_attn import flash_attn_varlen_func  # type: ignore[no-redef]
-    is_vllm_fa = False
+        except ImportError:
+            logger.debug(
+                "flash_attn not available on ROCm; "
+                "MLA models using TRITON_MLA will require flash_attn. "
+                "AITER_MLA backends use aiter kernels instead."
+            )
 
 
 def dynamic_per_batched_tensor_quant(
@@ -1917,6 +1927,12 @@ class MLACommonImpl(MLAAttentionImpl[M], Generic[M]):
             self._run_prefill_new_tokens = self._run_prefill_new_tokens_cudnn
             self._pad_v = False
         else:  # Use FlashAttention
+            if flash_attn_varlen_func is None:
+                raise RuntimeError(
+                    "MLA attention requires FlashAttention but it is not "
+                    "available. Please install flash_attn or use "
+                    "--attention-backend ROCM_AITER_MLA."
+                )
             logger.info_once("Using FlashAttention prefill for MLA", scope="local")
             self._run_prefill_context_chunk = self._run_prefill_context_chunk_fa
             self._run_prefill_new_tokens = self._run_prefill_new_tokens_fa