Text Generation with Transformer Decoder example (#914)

* added example for text generation with transformer decoder

* made model more lightweight

* restructured and updated example

* removed filler

* changed to perplexity metric

* added vocab training, made model input variable and other fixes

* copywriting fixes

* copywriting fixes 2

* copywriting fixes 3

* Copyedits

* formatting changes

* add_example files

* updated email of author for cla

* fixed whitespace

* changes removing error and quicker training

* suppressed warnings

* removed unused hyperparameter

* Title update

Co-authored-by: François Chollet <francois.chollet@gmail.com>
Co-authored-by: jessechancy <jessechanwork@gmail.com>
Co-authored-by: jessechancy <jessechan@google.com>