[C10D] Introduce C++ side Collective Callbacks. (#110307)
C++ side callbacks allow for advance users to get
access to the collective firehose.
It's worth mentioning and discussing the dire environment that those
callbacks are invoked. From either main thread of watchdog thread and
with a PTD lock held.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110307
Approved by: https://github.com/fduwjj
ghstack dependencies: #111061, #111072
