Bfloat16 zero2 (#1398)
* Changes for bfloat16 Zero2
* Cleaned up additional comments and debugging code
* Adapted fp16_master_weights_and_grads option to cover BF16
* Reverted fp16_master_weights_and_gradients extension to BFloat16 and minor cleanup
* Fixed formatting and variable naming errors recognized in testing
* Added relevant unit tests for bfloat16 with ZeRO-2
* Updates conditions for skipping BFloat16 unit tests
* Added check for NCCL inconsistent version naming convention
* Update skip message for Bfloat16 tests to mention additional checks
Co-authored-by: Olatunji Ruwase <olruwase@microsoft.com>
