[Bugfix] Exclude language_model_only key from MM AOT compile hash but include in model one (#34508)

Signed-off-by: Roger Wang <hey@rogerw.io>

tests/config/test_multimodal_config.py

@@ -3,6 +3,7 @@
 
 import pytest
 
+from vllm.config.model import ModelConfig
 from vllm.config.multimodal import MultiModalConfig
 from vllm.v1.attention.backends.registry import AttentionBackendEnum
 
@@ -23,3 +24,20 @@ def test_mm_encoder_attn_backend_hash_updates():
         mm_encoder_attn_backend=AttentionBackendEnum.FLASH_ATTN
     ).compute_hash()
     assert base_hash != overridden_hash
+
+
+def test_language_model_only_does_not_affect_mm_hash():
+    """language_model_only does not affect the ViT computation graph,
+    so it should not change the multimodal config hash."""
+    base_hash = MultiModalConfig().compute_hash()
+    lm_only_hash = MultiModalConfig(language_model_only=True).compute_hash()
+    assert base_hash == lm_only_hash
+
+
+def test_language_model_only_affects_model_hash():
+    """language_model_only affects the LM computation graph,
+    so it should change the model config hash."""
+    model = "llava-hf/llava-1.5-7b-hf"
+    base_hash = ModelConfig(model).compute_hash()
+    lm_only_hash = ModelConfig(model, language_model_only=True).compute_hash()
+    assert base_hash != lm_only_hash