[Reland] Add OnCompletion Hook to ProcessGroup (#106988) (#107233)
This allows infra/trainers to get detailed stats about communication
efficiencies without know anything about what model or distributed
training paradigms have been used. This is helpful as infra/trainer
package usually prefers to be as model/algorithm agnostic as possible.
Therefore, we cannot assume that infra/trainer can have access to all
collectives used by the model authors.
This commit adds an `OnCompletion` hook to `ProcessGroupNCCL` which
will be fired on every work completion event.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107233
Approved by: https://github.com/kumpera
