Add storage backends in init file (#325)

mmengine/fileio/__init__.py

 # Copyright (c) OpenMMLab. All rights reserved.
-from .file_client import BaseStorageBackend, FileClient
+from .file_client import (BaseStorageBackend, FileClient, HardDiskBackend,
+                          HTTPBackend, LmdbBackend, MemcachedBackend,
+                          PetrelBackend)
 from .handlers import BaseFileHandler, JsonHandler, PickleHandler, YamlHandler
 from .io import dump, load, register_handler
 from .parse import dict_from_file, list_from_file
 
 __all__ = [
-    'BaseStorageBackend', 'FileClient', 'load', 'dump', 'register_handler',
-    'BaseFileHandler', 'JsonHandler', 'PickleHandler', 'YamlHandler',
-    'list_from_file', 'dict_from_file'
+    'BaseStorageBackend', 'FileClient', 'PetrelBackend', 'MemcachedBackend',
+    'LmdbBackend', 'HardDiskBackend', 'HTTPBackend', 'load', 'dump',
+    'register_handler', 'BaseFileHandler', 'JsonHandler', 'PickleHandler',
+    'YamlHandler', 'list_from_file', 'dict_from_file'
 ]