[GLE] Explicitly build the training client using the GLE server handle

A graph object is returned after the GLE servers are launched, which is further used to build the client that the training worker uses to communicate with the GLE servers. While in distributed training（multiple training workers), we need to first get the handle of the graph (including the schema info, service IP and port), and use these information to build client in the training side. This issue is to add APIs and implementations to ease the process of explicitly building the training client using the GLE server handle.