Extract CompilationConfig from config.py (#22524)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/config/utils.py

@@ -0,0 +1,29 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from typing import TYPE_CHECKING, TypeVar
+
+if TYPE_CHECKING:
+    from _typeshed import DataclassInstance
+
+    ConfigType = type[DataclassInstance]
+else:
+    ConfigType = type
+
+ConfigT = TypeVar("ConfigT", bound=ConfigType)
+
+
+def config(cls: ConfigT) -> ConfigT:
+    """
+    A decorator that ensures all fields in a dataclass have default values
+    and that each field has a docstring.
+
+    If a `ConfigT` is used as a CLI argument itself, the `type` keyword argument
+    provided by `get_kwargs` will be
+    `pydantic.TypeAdapter(ConfigT).validate_json(cli_arg)` which treats the
+    `cli_arg` as a JSON string which gets validated by `pydantic`.
+
+    Config validation is performed by the tools/validate_config.py
+    script, which is invoked during the pre-commit checks.
+    """
+    return cls