[Gradient Compression] Remove some low-level methods of GradBucket class (#53098)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53098
Remove some low-level methods that are no longer needed since `get_per_parameter_tensors` method is added to `GradBucket` class.
Avoid unnecessary exposure to the internals before publishing GradBucket APIs.
ghstack-source-id: 122979064
Test Plan: buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_powerSGD_ddp_comm_hook_nccl
Reviewed By: osalpekar
Differential Revision: D26784249
fbshipit-source-id: d1b27bb026989c25a5b65be4767cb752afd6f19b
