[ROCm][CI] Fix cross-attention dispatch for encoder-decoder models (#38450)

Signed-off-by: Andreas Karatzas <akaratza@amd.com>

tests/entrypoints/openai/speech_to_text/test_transcription_validation_whisper.py

@@ -14,13 +14,62 @@ import pytest_asyncio
 import soundfile as sf
 
 from tests.utils import RemoteOpenAIServer
+from vllm.platforms import current_platform
 
 MODEL_NAME = "openai/whisper-large-v3-turbo"
 
+# Disable prefix caching on ROCm to reduce non-determinism in
+# streaming-vs-non-streaming comparisons.
+_ROCM_ARGS = ["--no-enable-prefix-caching"] if current_platform.is_rocm() else []
 
-@pytest.fixture(scope="module")
-def server():
-    with RemoteOpenAIServer(MODEL_NAME, []) as remote_server:
+
+def _get_attention_backend_params() -> list[str | None]:
+    """Return attention backends to parametrize the server fixture with.
+
+    On ROCm, we test multiple backends explicitly:
+      - None: default auto-selection (ROCM_ATTN for decoder self-attention,
+              falls back to ROCM_AITER_UNIFIED_ATTN or TRITON_ATTN for
+              cross-attention since ROCM_ATTN doesn't support ENCODER_DECODER)
+      - TRITON_ATTN: always available on ROCm
+      - ROCM_AITER_UNIFIED_ATTN: only on gfx942/gfx950
+
+    On non-ROCm platforms, we just run with the default backend.
+    """
+    try:
+        from vllm.platforms import current_platform
+
+        if current_platform.is_rocm():
+            backends: list[str | None] = [None, "TRITON_ATTN"]
+            from vllm.platforms.rocm import _ON_MI3XX
+
+            if _ON_MI3XX:
+                backends.append("ROCM_AITER_UNIFIED_ATTN")
+            return backends
+    except Exception:
+        pass
+    return [None]
+
+
+# Aiter backends need VLLM_ROCM_USE_AITER=1 (and MHA=1 for ROCM_AITER_FA)
+# to be enabled in the server subprocess.
+_AITER_ENV = {
+    "VLLM_ROCM_USE_AITER": "1",
+    "VLLM_ROCM_USE_AITER_MHA": "1",
+}
+
+_ATTN_BACKENDS = _get_attention_backend_params()
+_ATTN_IDS = [b or "default" for b in _ATTN_BACKENDS]
+
+
+@pytest.fixture(scope="module", params=_ATTN_BACKENDS, ids=_ATTN_IDS)
+def server(request):
+    args = [*_ROCM_ARGS]
+    env_dict = None
+    if request.param is not None:
+        args += ["--attention-backend", request.param]
+        if "AITER" in request.param:
+            env_dict = _AITER_ENV
+    with RemoteOpenAIServer(MODEL_NAME, args, env_dict=env_dict) as remote_server:
         yield remote_server