Add a vocab training utility for word piece

[mattdangerw]

We would like to add a vocab training utility for wordpiece. This can leverage the utilities for doing this in tensorflow text.

Note that we do not have to cover all the use cases here, or be performant for all data sizes. We should just aim to build a useful tool that covers than majority of common training use cases.

[mattdangerw]

@jessechancy will be working on this!

[mattdangerw]

This is done!