Refactorize critics and decoders

[Plamen Valentinov Kolev]

As we added a new Decoder, we can see that the code it's quiet similar and it should be refactorized in order to follow the same structure as encoders.py.
Same it's happening for critics.py. Refactorizing this now, will save us time in unit tests and if we want to add more architectures in the future it will be easier.