[Model] New model support for microsoft/Phi-4-mini-flash-reasoning (#20702)

Signed-off-by: Congcong Chen <congcongchen@microsoft.com>

tests/models/test_initialization.py

@@ -103,6 +103,9 @@ def test_can_initialize(model_arch: str, monkeypatch: pytest.MonkeyPatch):
                        _initialize_kv_caches_v1), monkeypatch.context() as m):
         if model_info.v0_only:
             m.setenv("VLLM_USE_V1", "0")
+        if model_arch == "Phi4FlashForCausalLM":
+            # Phi4FlashForCausalLM only supports DIFFERENTIAL_FLASH_ATTN backend
+            m.setenv("VLLM_ATTENTION_BACKEND", "DIFFERENTIAL_FLASH_ATTN")
         LLM(
             model_info.default,
             tokenizer=model_info.tokenizer,