Clearly factor out the different code paths used for different input modes in...

Clearly factor out the different code paths used for different input modes in `fit`, `evaluate`, and `predict`. This makes it clearer which arguments are supported in each case, and which validation checks or preprocessing steps are being applied in each case.

The refactor revealed that `validation_split` was not supported with distribution strategies (not previously checked).

PiperOrigin-RevId: 226366415