New fedavg fedprox

As discussed in the meeting today, I modified the notebook FedAvg and FedProx and create_MNIST.py so to compare real MNIST iid with real MNIST non-iid. I also included at the end of the notebook an extra section for the synthetic MNIST non-iid as in the original notebook.

Please give it a look if it's ok for you @mlorenzi Yann

The link to the notebook: https://gitlab.inria.fr/ssilvari/flhd/-/blob/new_fedavg_fedprox/federated_learning/FedAvg_FedProx_MNIST_iid_and_noniid.ipynb