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Summary:
- Use dictionary to select the `RNNTBundle` and the corresponding dataset.
- Use the dictionary's keys as choices in ArgumentParser

Pull Request resolved: https://github.com/pytorch/audio/pull/2239

Reviewed By: mthrok

Differential Revision: D34267070

Pulled By: nateanl

fbshipit-source-id: 99c7942d5c7c1518694e1ae02a55a7decd87c220