Add runtime_context to get some runtime fields in worker (#4065)

python/ray/__init__.py

@@ -92,6 +92,7 @@ import ray.internal  # noqa: E402
 # some functions in the worker.
 import ray.actor  # noqa: F401
 from ray.actor import method  # noqa: E402
+from ray.runtime_context import _get_runtime_context  # noqa: E402
 
 # Ray version string.
 __version__ = "0.7.0.dev0"
@@ -103,6 +104,7 @@ __all__ = [
     "WORKER_MODE",
     "__version__",
     "_config",
+    "_get_runtime_context",
     "actor",
     "connect",
     "disconnect",