Add Semantic Similarity with BERT example (#205)

* Adding semantic similarity example

* added all the necessary changes

* added new necessary changes

* removed step_per_epoch

* removed steps_per_epoch

* reduced no. of epochs

* Copediting

* added mutiprocessing in fit

* updated fine-tuning

* Copyediting

* added generated files

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

Author: MohamadMerchant