Add a layer to dynamically generate mask

[chenmoneygithub]

Is your feature request related to a problem? Please describe.
Masking is an essential part in LM models, so we need to have a layer to do the work.

Describe the solution you'd like
Create a new layer, which takes in sequence data (could be both RaggedTensor and Tensor), and returns masked sequence, and masked positions and ids.

Describe alternatives you've considered
We can directly use tf_text, but the API is not very user-friendly.

Additional context
N/A

[mattdangerw]

This is done! #61