* refactor low level zero
* fix zero2 and support cpu offload
* avg gradient and modify unit test
* refactor grad store, support layer drop
* refactor bucket store, support grad accumulation
* fix and update unit test of zero and ddp
* compatible with tp, ga and unit test
* fix memory leak and polish
* add zero layer drop unittest
* polish code
* fix import err in unit test
* support diffenert comm dtype, modify docstring style
* polish code
* test padding and fix
* fix unit test of low level zero
* fix pad recording in bucket store
* support some models
* polish