Fixes a number of usability issues with model_to_estimator, in particular:

- make it possible to use a model that was compiled with a TF optimizer (do not require a Keras optimizer)
- do not require input to be dict (input_fn supports plain arrays)
- do not require `config` to be a RunConfig instance, can now be a dict (better UX)
- make it possible to use a subclassed model (caveat: weights are not preserved, yet)
- clear error message when model isn't compiled; improve various error messages

PiperOrigin-RevId: 187959927