Add logging for ProcessGroup backends. (#73702)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73702
This PR adds logging to track usage of ProcessGroup backends. The
change involves adding an `init()` method to the ProcessGroup base class and
modifying all the subclasses to call it.
We can't add logging directly in the ProcessGroup constructor since
`getBackendName()` is pure virtual and can't be called from the base class
constructor. See
https://isocpp.org/wiki/faq/strange-inheritance#calling-virtuals-from-ctors for
more details.
ghstack-source-id: 150381852
Test Plan: check logging
Reviewed By: cbalioglu
Differential Revision: D34596295
fbshipit-source-id: 5baa7bfb53bdcd1b4032292c4e7715467f983288
(cherry picked from commit b2344144fe0c25db02c2e958f2acd772e9cd4dc8)
