[ROCm][Bugfix] Use BF16 MLA prefill for short prompts

Avoid FP8 rounding perturbations on short greedy-decoding requests while preserving the optimized AITER path for long-context prefill.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: LiuYinfeng01

vllm/v1/attention/backends/mla/rocm_aiter_mla.py

@@ -1035,7 +1035,9 @@ def forward_mha(
         prefill_metadata = attn_metadata.prefill
         has_context = prefill_metadata.chunked_context is not None
 
-        if has_context:
+        # FP8 prefill can perturb greedy decoding for short prompts, where its
+        # speedup is negligible. Retain it for long, context-free prefill.
+        if prefill_metadata.max_query_len < 1024 or has_context:
             return super().forward_mha(
                 q,
                 kv_c_normed,