Enable federated learning with DeepCoxPH

[matteo4diani]

Federated learning frameworks like https://github.com/adap/flower are of particular interest to healthcare research, to work seamlessly with auton-survival the following features are needed:

• Easy and clean access to the model's PyTorch component
• PyTorch module pre-initialisation before fit
• Make the Breslow estimator optional as it contains too much info about the dataset to be safe in a federated setup