remove bucket_size_limit property from bucket struct
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73731
during rebuilting bucket, in addition to sync bucket_indice, per_bucket_limits should be synced as well before calling initialize_buckets(). Syncing per_bucket_limits will increase communicaton volume as well increasing code complexity, after taking a further look at the codes, per_bucket_limits used inside initialize_buckets() is actually not useful, it assigns bucket_size_limit property to bucket struct, but the property is not used anywhere. So it is good to remove this property and avoid syncing per_bucket_limits.
Differential Revision: [D34605513](https://our.internmc.facebook.com/intern/diff/D34605513/)
Approved by: https://github.com/rohan-varma
