docs(mge/distributed): add document for distributed.backend

GitOrigin-RevId: 6cdcf7af7796e551a3bbaca9f84531fd84791db8

imperative/python/megengine/distributed/__init__.py

@@ -31,6 +31,18 @@ from .server import Client, Server
 
 @mproperty
 def backend(mod):
+    r"""
+    Get or set backend of collective communication.
+    Available backends are ['nccl', 'shm', 'rccl']
+
+    Examples:
+
+    .. code-block::
+
+        import megengine.distributed as dist
+        dist.backend = "nccl"
+
+    """
     assert group._sd, "please call init_process_group first"
     return group._sd.backend