Update serving signatures and make sure we actually use them
This PR does some stuff to lay the groundwork for my plan to focus on TF model deployment:

- Updates the signatures on all our `model.serving` methods to use `int64` instead of `int32`
- Overrides `model.save()` so that the default signature for our models is now `model.serving`
- Casts all `int32` inputs to our TF models to `int64` in `input_processing`

The net effect is to standardize the int dtypes we use, and also make `model.save()` actually save a usable trace (although users can of course still override it with their own signatures if needed).
_The documentation is not available anymore as the PR was closed or merged._