Lift stats generator: use Tuple instead of FeaturePath when encoding feature paths as keys in a PTable.
Lift stats generator: use Tuple instead of FeaturePath when encoding feature paths as keys in a PTable.

Also changed the output type of some internal functions from numpy arrays to python lists. elements of an numpy array are of numpy types which do not have deterministic beam coders.

PiperOrigin-RevId: 364593214

tensorflow/tfx,tensorflow/tfx