Add an AlbertMaskedLM task model

[mattdangerw]

In #653 we added a masked language modeling task for RoBERTa. We can make a similar change for the Albert model.

• Update AlbertTokenizer to expect a mask token.
• Add a AlbertMaskedLMPreprocessor preprocessor layer and tests.
• Add a AlbertMaskedLM task model and tests.
• Update keras_nlp/models/__init__.py to export AlbertMaskedLM and AlbertMaskedLMPreprocessor.

[kanpuriyanawab]

I would love to work on this issue!