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Updates for ZeRO stage 2 + ZeRO stage 1 w. RS
Co-authored-by: Tunji Ruwase <olruwase@microsoft.com>
Co-authored-by: Samyam Rajbhandari <samyamr@microsoft.com>
Co-authored-by: Shaden Smith <ShadenTSmith@gmail.com>
Co-authored-by: Elton Zheng <eltonz@microsoft.com>
Co-authored-by: Shaden Smith <Shaden.Smith@microsoft.com>
Co-authored-by: yuxionghe <yuxhe@microsoft.com>
Co-authored-by: Arash Ashari <arashari@microsoft.com>

* Enhancement: Ability to load checkpoint without loading the optimizer states. Unittest testing saving and loading checkpoint with fused, unfused and zero optimizer. The unitest takes about 165s