Major refactoring of Model classes (Step 2)

[alanakbik]

As we are now adding many more types of models to Flair (see #2333) we are refactoring the logic of Flair model classes to remove many redundancies and make the code hopefully easier to read.

This PR is the second step in this refactoring process and mainly adds the new DefaultClassifier base class to Flair. It contains all logic that is shared by models that do classification, so that these models use the same (1) evaluation code, (2) prediction code and (3) loss computation code. Any Flair model that inherits from DefaultClassifier now only has to implement the forward_pass() method - all the rest is then supplied by the parent class.

With this refactoring, the TextClassifier, the RelationExtractor, the TextPairClassifier and the SimpleSequenceTagger now inherits from DefaultClassifier. Accordingly, much redundant code has disappeared. The SequenceTagger still inherits from Classifier, but this will change as a major refactoring here is planned as well.

In addition, the TARS classes (TARSClassifier and TARSTagger) were broadly refactored so thatt they inherit from the new base class FewshotClassifier and share most of the TARS logic through the super class. This removed many redundancies. The implementation of TARSClassifier is now slightly different from before as single-label predictions are no longer enforced.

In addition, many small changes were made:

• The batch_size parameter in TransformerDocumentEmbeddings has been removed since it is very counterintuitive and caused speed losses if not used correctly. The default behavior is not to batch over all sentences in a given mini-batch.
• You can now remove entries from a Dictionary with the method remove_item()
• The sanity checks for setting scores on Labels have been removed
• Potentially breaking: the make_label_dictionary() method of the Corpus now requires you to specify a label_type. To make this change easier, it now prints a list of all available label types when a dictionary is computed
• The label_type in the GO_EMOTIONS dataset is renamed to 'emotion'
• The TextPairClassifier now resides in its own module
• The flair.nn module was split into a folder structure