A gRPC-related suggestion

[joneswong]

In the current main branch (commit 954322c), the implementation of the gRPCCommManager class (see federatedscope/core/communication.py) largely refers to that in FedML (see https://github.com/FedML-AI/FedML/blob/master/fedml_core/distributed/communication/gRPC/grpc_comm_manager.py in commit 0fb63dd157e55ee603b7049568bf4c4ed0586e71), as commented in FederatedScope's codebase. This class is based on gRPC, a modern open source high performance Remote Procedure Call (RPC) framework. A gRPCCommManager (i) keeps addresses of potential message receivers in a dict/list collection and (ii) has wrapper functions that call APIs in gRPC for message sending and receiving. Many similar variants of such wrapper functions have been widely adopted in related packages. Although FederatedScope obeys the Apache-2.0 License and had a declaration of FedML's copyright, in order to avoid the risk of unintended infringement and unnecessary disputes with FedML, we re-implement this class by refering to the examples in gRPC tutorial in this commit.