Update Keras Mixin (#284)
* :art: add Keras mixin to root level __init__.py
* :art: remove kwargs from _save_pretrained
* :construction: wip open heart surgery on keras mixin/utils
* :construction: wip
* :white_check_mark: update tests
* :lipstick: style
* :white_check_mark: work on tests
* :bug: require tf in keras functional tests
* :bug: add placeholders for dummy models
* :white_check_mark: Update test params to use cloned model
* :white_check_mark: fix tests
* :white_check_mark: move tf clone model inside test functions
* :lipstick: style
* :white_check_mark: test save does not work if keras model not built
* :art: add explicit endpoint
* :fire: remove class attrs I decided not to use
* :construction: wip
* :white_check_mark: clean up keras integration tests
* :art: make sure functional API includes config
* :construction_worker: add tensorflow build to CI
* Apply suggestions from code review
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
* :construction: wip
* :construction: wip
* :truck: rename model.hf_config to model.config
* :fire: rm some code
* :pencil2: Updates from code review
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
