[PT-D][Sharding] Make Partial separate file and enable padding for reshard when size not divisible by world_size (#73392)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73392
As title, we make the following changes to partial tensor:
1. We make partial tensor a dedicate file so that we can decouple it from sharded tensor logic for better code structure and maintenance.
2. We enabled the padding for the local tensor when its size is not divisible by `world_size`.
ghstack-source-id: 149907802
Test Plan: CI+lint
Reviewed By: pritamdamania87
Differential Revision: D34459863
fbshipit-source-id: 6bfb5dd322ecba0d085916e56b634b0c748b83e8
(cherry picked from commit ceefadb3fb7c4cc7487d53b401f5c25c16fd04d5)
