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

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

vllm/v1/attention/backends/rocm_aiter_fa.py

@@ -758,11 +758,12 @@ class AiterFlashAttentionBackend(AttentionBackend):
 
     @classmethod
     def supports_attn_type(cls, attn_type: str) -> bool:
-        """ROCM AITER FA supports decoder and encoder-decoder (cross) attention."""
-        return attn_type in (
-            AttentionType.DECODER,
-            AttentionType.ENCODER_DECODER,
-        )
+        """ENCODER_DECODER is not supported because the prefill path uses
+        flash_attn_varlen_func with cu_seqlens_k set to decoder
+        query_start_loc (not encoder seq lens) and causal=True, both of
+        which are incorrect for cross-attention layers.
+        """
+        return attn_type in (AttentionType.DECODER,)
 
     @staticmethod
     def get_supported_kernel_block_sizes() -> list[int | MultipleOf]:

vllm/v1/attention/backends/rocm_attn.py

@@ -212,12 +212,17 @@ class RocmAttentionBackend(AttentionBackend):
 
     @classmethod
     def supports_attn_type(cls, attn_type: str) -> bool:
-        """RocmAttention supports all attention types."""
+        """ENCODER_DECODER is not supported because
+        chunked_prefill_paged_decode's prefill kernel (context_attention_fwd)
+        assumes self-attention semantics: it treats passed K/V as new tokens
+        to mix with cached K/V. For cross-attention layers the encoder K/V
+        are already fully cached, so mixing them again produces incorrect
+        results when max_query_len > 1 (e.g. beam search).
+        """
         return attn_type in (
             AttentionType.DECODER,
             AttentionType.ENCODER,
             AttentionType.ENCODER_ONLY,
-            AttentionType.ENCODER_DECODER,
         )
 
     @staticmethod