docs(distributed.functional): add return type for all_reduce_max (jira #MGE-2706)

GitOrigin-RevId: a29f1f1880

imperative/python/megengine/distributed/functional.py

@@ -271,6 +271,7 @@ def all_reduce_max(
     :param inp: input tensor.
     :param group: communication group.
     :param device: execution device.
+    :returns: reduced tensor. 
     """
     mode = CollectiveComm.Mode.ALL_REDUCE_MAX
     return collective_comm(inp, mode, group, device)