TF model cards (#14720)
* Initial commit for Keras model cards
* Revert accidental change
* make style
* make style
* make style
* Fix PR comments
* Move repo creation to __init__
* Fixes to README.md creation
* Partial progress for proper card creation on `push_to_hub`
* Proper card creation from `push_to_hub` plus fixes for malformed model cards
* Fixes for model card creation outside the callback
* Adding a model card creation test
* Putting the model card creation test in the right file.
Good job, Matt.
* make style
* Fix model card test temp dir usage
* Fix model card creation when no optimizer present
* Fixes for when training history not present
* Fix accidental edit to test_modeling_common
