Fix ProcessGroupMPITest.cpp Gather, Scatter and SendRecv. Enable ProcessGroupMPITest (#56709)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56709
Right now, ProcessGroupMPITest testGather() fails with
```
what(): Gather: number of output tensors should be 0 for non-root
[devgpu025:429730] *** Process received signal ***
```
there is a similar issue with testScatter() where number of input/output tensors on source/destination respectively should be 0.
In addition testSendRecv(true); fails with
```
terminate called after throwing an instance of 'std::runtime_error'
what(): src rank is wrong for recvAnysource
```
since we never populate `srcRanks`
Test Plan: Imported from OSS
Reviewed By: pbelevich
Differential Revision: D28001963
Pulled By: agolynski
fbshipit-source-id: c381dfc6f417ee78fbbaf884e567b0485076dfc8
