Move the inline implementations of GradBucket class to the header. (#44339)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44339
Moved the inline implementations of GradBucket class to the header for
succinctness and readability. This coding style is also consistent with
reducer.h under the same directory.
Test Plan: buck test caffe2/torch/lib/c10d:ProcessGroupGlooTest
Reviewed By: pritamdamania87
Differential Revision: D23569701
fbshipit-source-id: 237d9e2c5f63a6bcac829d0fcb4a5ba3bede75e5
