some refactor work of parameter server

[QiJune]

Step 1:

• [ ] embedding table Python data structure
• [ ] tensor proto message and Python data structure
• [ ] a new PS, which binds to a k8s service
• [ ] parameter sharding hash function

Step 2:

• [ ] KVStore Python data structure
• [ ] define PS service interface in proto
• [ ] refine current OptimizerWrapper to support KVStore

Step 3:

• [ ] refine worker related code to cooperate with new PS design
• [ ] evaluation support
• [ ] checkpoint support

Step 4:

• [ ] extend one-PS node to multi-PS node
• [ ] PS replica support

Step 5:

• [ ] fault tolerate feature and test