Update ProcessGroup collective C++ APIs to be non-pure virtual functions (#64943)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64943
Most ProcessGroup Collective APIs are pure virtual. As a result, c10d extensions need to override all of them and throw an error if they don't need certain APIs. This is too verbose for users. This commit changes those collective APIs to virtual and throws an error by default. Note that ProcessGroup is still an abstract class as `getBackendName` is a pure virtual function that all subclasses have to override.
cc pietern mrshenli pritamdamania87 zhaojuanmao satgera rohan-varma gqchen aazzolini osalpekar jiayisuse SciPioneer H-Huang cbalioglu gcramer23
Test Plan: Imported from OSS
Reviewed By: cbalioglu
Differential Revision: D30906866
Pulled By: mrshenli
fbshipit-source-id: c4df8962d60350a44d2df72fd04f9dd6eadb9fa6
