[Core] Consolidate prompt arguments to LLM engines (#4328)

Co-authored-by: Roger Wang <ywang@roblox.com>

tests/test_utils.py

@@ -0,0 +1,63 @@
+import pytest
+
+from vllm.utils import deprecate_kwargs
+
+from .utils import error_on_warning
+
+
+def test_deprecate_kwargs_always():
+
+    @deprecate_kwargs("old_arg", is_deprecated=True)
+    def dummy(*, old_arg: object = None, new_arg: object = None):
+        pass
+
+    with pytest.warns(DeprecationWarning, match="'old_arg'"):
+        dummy(old_arg=1)
+
+    with error_on_warning():
+        dummy(new_arg=1)
+
+
+def test_deprecate_kwargs_never():
+
+    @deprecate_kwargs("old_arg", is_deprecated=False)
+    def dummy(*, old_arg: object = None, new_arg: object = None):
+        pass
+
+    with error_on_warning():
+        dummy(old_arg=1)
+
+    with error_on_warning():
+        dummy(new_arg=1)
+
+
+def test_deprecate_kwargs_dynamic():
+    is_deprecated = True
+
+    @deprecate_kwargs("old_arg", is_deprecated=lambda: is_deprecated)
+    def dummy(*, old_arg: object = None, new_arg: object = None):
+        pass
+
+    with pytest.warns(DeprecationWarning, match="'old_arg'"):
+        dummy(old_arg=1)
+
+    with error_on_warning():
+        dummy(new_arg=1)
+
+    is_deprecated = False
+
+    with error_on_warning():
+        dummy(old_arg=1)
+
+    with error_on_warning():
+        dummy(new_arg=1)
+
+
+def test_deprecate_kwargs_additional_message():
+
+    @deprecate_kwargs("old_arg", is_deprecated=True, additional_message="abcd")
+    def dummy(*, old_arg: object = None, new_arg: object = None):
+        pass
+
+    with pytest.warns(DeprecationWarning, match="abcd"):
+        dummy(old_arg=1)