[Bugfix][Frontend] validate arg priority in frontend LLM class before add request (#27596)

Signed-off-by: Junpu Fan <junpufan@gmail.com>

tests/entrypoints/llm/test_generate.py

@@ -71,6 +71,26 @@ def test_multiple_sampling_params(llm: LLM):
     assert len(PROMPTS) == len(outputs)
 
 
+def test_multiple_priority(llm: LLM):
+    # Generate works when priority is None
+    outputs = llm.generate(PROMPTS, sampling_params=None, priority=None)
+    assert len(PROMPTS) == len(outputs)
+
+    # Generate works when length of priority is same as the len(PROMPTS)
+    outputs = llm.generate(PROMPTS, sampling_params=None, priority=[0] * len(PROMPTS))
+    assert len(PROMPTS) == len(outputs)
+
+    # Exception raised, if the length of priority does not match the length of prompts
+    with pytest.raises(ValueError):
+        outputs = llm.generate(
+            PROMPTS, sampling_params=None, priority=[0] * (len(PROMPTS) - 1)
+        )
+
+    # Exception raised, if the priority list is empty
+    with pytest.raises(ValueError):
+        outputs = llm.generate(PROMPTS, sampling_params=None, priority=[])
+
+
 def test_max_model_len():
     max_model_len = 20
     llm = LLM(