Restructuring prof dag counters (#13321)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/13321
This diff simply refactors the `ProfDAGCounters` into two:
* `ProfDAGCounters` that gathers stats at runtime.
* `ProfDAGReport` which holds the report from the gathered stats once stats collection is done.
This refactoring allow us to implement `+=` for `ProfDAGReport`, which can be used for aggregating same-net reports on each host.
Reviewed By: donglimm
Differential Revision: D12837988
fbshipit-source-id: 0470c5fd6437f12711cab25a15a12965d79b2a91
