Add PowerSGD compressor for AllReduce based strategy

[ZeyaWang]

System information

• AutoDist version: 0.7.0
• Are you willing to contribute it (Yes/No): Yes

Describe the new feature and the current behavior/state
Currently, the supported compressors for the AllReduce based strategies do not include PowerSGD compressor, which is an important compression algorithm proposed in arxiv.org/abs/1905.13727. It would be helpful to include this compressor.

Will this change the current API? How?
It will add one more option to the compressors.

Describe alternatives you've considered
N/A

Additional context
N/A

[zhisbug]

@ZeyaWang : are you working on this one now?