Implement Robust Federated Aggregation in emnist_with_targeted example

[Anupam Mediratta]

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We want to implement Robust Federated Averaging in [1] into emnist_with_targeted_attack.

The readme in the link contains how to use it. But you have to tinker a bit.

We then want the same metrics when RFA is used.

1. https://github.com/tensorflow/federated/tree/master/tensorflow_federated/python/research/robust_aggregation