Split _validate_trainable_layers into validator and num_of_layers selector

[jdsgomes]

We use _validate_trainable_layers as a validator but also as a way to determine the actual number of trainable layers based on default values, pertained flag etc.

• It would be clearer to have separate checks for validation and another method to set the actual value instead of having a checker with side effects.
• Check if it makes sense to add the default value as a parameter default value instead of None?
• Should we use ValueError exception instead of assert?

cc @jdsgomes

cc @datumbox