* Masked language modeling example

* format code

* train_step workflow and utility func implemented

* mlm and fine tune with bert from scratch

I have implemented Mask Language Model and Fine-tune model with bert from scratch using tf-nightly

* updated according to review feedback

* file formatted

* text paragraph added and refactored

* Copyediting, fix fine-tuning logic

* refactored and formatted

* refactored accordingly

* Copyediting, refactor loss_fn

* generated files added

* formating

Co-authored-by: François Chollet <francois.chollet@gmail.com>