Improve Model and its subclasses.

Clean-up Model and its subclasses:

• Improve documentation of Model methods.
• Harmonize _unpack_batch method name for the current Model subclasses.
• Harmonize loss-averaging behavior of the current Model subclasses.

Improve TensorflowModel:

• Optimize gradients computation in TensorflowModel using tf.function.
• Improve the management and serialization of loss in TensorflowModel.
• Implement utilities to build keras Loss objects from strings or functions.