we shouldn't be saving any new models if running in evaluation mode.

[ishaileshpant]

Current behavior:
In evaluation run of the federation the aggregator saves the model protobuf states (best and last pbuf files) which is not relevant for evaluation run as no training is performed.

Affected Version:
1.7

Expected behavior:

We shouldn't save any model proto buf files only an init.pbuf should be sufficient to run a evaluation run and make evaluation run idempotent from model state perspective