[Gradient Compression] Don't provide default values in GradBucket constructor (#53102)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53102
In `GradBucket` constructor, `offsets`, `lengths`, and `sizes_vec` are optional arguments and could possibly be empty. It will be safe to remove the default values.
ghstack-source-id: 122833603
Test Plan: waitforbuildbot
Reviewed By: rohan-varma
Differential Revision: D26748199
fbshipit-source-id: 2e3bcd1b732851919a64bbbd20fe85e77a616fe3
