Separate out Keras API generation from TensorFlow.

When moving Keras to a separate repo, we will have to separate the API
generation from the rest of TensorFlow. The generated API files will have
to part of the Keras package, as core TensorFlow will have no way of knowing
which version of the tf.Keras API will be installed.

This CL does a few things...
- Move ALL @tf_export("keras.*") declarations in to keras/ directory.
  See python/keras/ops.py where I had to move lots of these decorators
  out of TensorFlow code.
- Change ALL @tf_export("keras.*") to @keras_export("keras.*"). Doing
  this will allow us to generate the public Keras python __init__.py files
  separately from the rest of the TensorFlow API.
- Add a new api_gen targets to keras/api/BUILD to generate this API.

PiperOrigin-RevId: 226050838