Added logging for the Reducer's non-member functions. (#65023)

Added logging for the Reducer's non-member functions. (#65023)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65023

Added an optional logging parameter for non-member functions `compute_bucket_assignment_by_size` and `verify_replica0_across_processes`. If a logger is provided then `TORCH_CHECK` assertions are replaced with a wrapper that logs the error to the DDP reducer's logger before calling `TORCH_CHECK`. If a logger is not provided `TORCH_CHECK` is still called.

Modified python-side calls to `_compute_bucket_assignment_by_size` and `_verify_model_across_ranks` to include a logger whenever possible. A notable exception is when these non-member functions are called in DDP's constructor - we cannot pass in a logger as they may have not been initialized yet.

We also added 4 new tests: `test_compute_bucket_assignment_by_size_sparse_error_{with, without}_logger` which tests the `_compute_bucket_assignment_by_size` function to ensure that sparse tensors are rejected and the errors are logged.  `test_verify_model_across_rank_{with, without}_logger` calls `_verify_model_across_ranks` to ensure that ill-formed models (different ranks have different number of parameters compared to rank 0) are rejected and the errors are logged. The test `test_ddp_model_diff_across_ranks` remains unchanged - while it does construct a ill-formed DDP instance which triggers the error in `_verify_model_across_ranks`, we cannot check the logger because this error appears in the constructor.

Lastly, did some cleanup of the `test_ddp_model_diff_across_ranks` function to make the logic of choosing which context manager and error message to use more clean.

Test Plan:
**Build commands**
`buck build mode/dev-nosan //caffe2/test/distributed:distributed_nccl_spawn --keep-going`

`buck build mode/dev-nosan //caffe2/test/distributed:distributed_gloo_spawn --keep-going`

**Test commands**
Test for `_compute_bucket_assignment_by_size` (Python)/ `compute_bucket_assignment_by_size` (C++)
`BACKEND={nccl, gloo} WORLD_SIZE=2 ../buck-out/dev/gen/caffe2/test/distributed/distributed_{nccl, gloo}_spawn#binary.par -r test_compute_bucket_assignment_by_size_sparse_error_{with, without}_logger`

Test for `_verify_model_across_ranks` (Python)/`verify_replicas0_across_process` (C++)
`BACKEND={nccl, gloo} WORLD_SIZE=2 ../buck-out/dev/gen/caffe2/test/distributed/distributed_{nccl, gloo}_spawn#binary.par -r test_verify_model_across_ranks_{with, without}_logger`

Test that constructs an ill-formed DDP instance. Only did cleanup of this function.
`BACKEND={nccl, gloo} WORLD_SIZE=2 ../buck-out/dev/gen/caffe2/test/distributed/distributed_{nccl, gloo}_spawn#binary.par -r test_ddp_model_diff_across_ranks`

Reviewed By: rohan-varma

Differential Revision: D30924790

fbshipit-source-id: dae6fa82485a204a6a4b022f2d073417d07ebb2f