Update on "[C10D] Support group ranks in P2POp and batch_isend_irecv"

Changes semantic of __repr__ of P2POp: s, d are now group ranks instead
of global ranks. I think this is OK since I also updated the field names
to make this obvious.

Also add mypy annotations

Partially addresses RFC 0042 (pytorch/rfcs#71)
See more details/motivation in #140460

cc H-Huang awgu kwen2501 wanchaol fegin fduwjj wz337 d4l3k c-p-i-o

[ghstack-poisoned]

torch/distributed/distributed_c10d.py

@@ -488,10 +488,10 @@ def __init__(
         """Init."""
         self.op = op
         self.tensor = tensor
+        self.group = _group_or_default_group(group)
         self.peer = _canonicalize_group_rank(
             self.group, peer, group_peer, return_global=True
         )
-        self.group = _group_or_default_group(group)
         self.tag = tag
         self.group_peer = _canonicalize_group_rank(self.group, peer, group_peer)