Polish 'Model' API and backend code.

This MR is an extension to MR !188 (merged), that addressed issue #472 (closed) about implementing a new 'Model' abstraction layer and refactoring framework-specific code from the current 'TrainingPlan' classes into it and its framework- or model-specific subclasses.

This MR implements the following changes:

• Disambiguate class attributes from instance attributes in 'Model' and its subclasses.
  • Declare instance attributes as part of __init__ methods and class attributes at the proper scope.
  • Use the ClassVar type-hint to further highlight that some attributes are defined at class level.
• Add Model._model_type private class attribute and use it for init-type type-checking.
  • This class attribute enables specifying what classes of models may be wrapped.
  • It was used to refactor type-checking code into the shared parent Model.__init__.
• Revise 'Model.get_weights' and 'Model.get_gradients' signatures.
  • Replace the generic return_type attribute with as_vector: bool = False.
  • This was done in accordance with @scansiz, with issue #481 (closed) in mind.
• Revise BaseSkLearnModel.train backend and remove superfluous private attributes.
• Revise TorchModel.init_params.
  • Previously, the signature and actual type of this attribute did not match.
• Perform an overall cleaning of import statements' ordering, type hints and code formatting.
• Rename scikit-learn backend Models dict to SKLEARN_MODELS out of PEP-8 compliance.