[Encoder Decoder] Update Mllama to run with both FlashAttention and XFormers (#9982)

Signed-off-by: Sourashis Roy <sroy@roblox.com>

tests/encoder_decoder/test_e2e_correctness.py

@@ -7,7 +7,7 @@ from typing import List, Optional, Tuple
 import pytest
 from transformers import AutoModelForSeq2SeqLM
 
-from vllm.attention.selector import (_Backend,
+from vllm.attention.selector import (_Backend, _cached_get_attn_backend,
                                      global_force_attn_backend_context_manager)
 from vllm.platforms import current_platform
 from vllm.sequence import SampleLogprobs
@@ -34,6 +34,13 @@ def vllm_to_hf_output(
     return output_ids, hf_output_str, out_logprobs
 
 
+@pytest.fixture(autouse=True)
+def clear_cache():
+    """Fixture to clear backend cache before each test."""
+    _cached_get_attn_backend.cache_clear()  # Clear the cache
+    yield  # This allows the test to run
+
+
 @pytest.mark.parametrize("model", ["facebook/bart-large-cnn"])
 @pytest.mark.parametrize("dtype", ["float"])
 @pytest.mark.parametrize("attn_backend", LIST_ENC_DEC_SUPPORTED_BACKENDS)