[Gradient Compression] Export sizes, along with length and offset of each variable to GradBucket for PowerSGD (#47203)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47203
1. Create a new field in BucketReplica to store sizes info for each variable.
2. Export sizes list, along with lengths and offsets to GradBuceket.
These fields are needed for PowerSGD.
Original PR issue: Investigate Applying PowerSGD to Communication Hook for Gradient Compression #47202
ghstack-source-id: 115875194
Test Plan: Checked the field values from log.
Reviewed By: rohan-varma
Differential Revision: D24644137
fbshipit-source-id: bcec0daf0d02cbf25389bfd9be90df1e6fd8fc56
