Allow discrete sequence lengths (enumerated input shapes)

When the Core ML model is converted using flexible shapes, it doesn't run on GPU or ANE. We are currently using a fixed sequence length, but a better solution (to explore) would be to:

- Use [`EnumeratedShapes`](https://apple.github.io/coremltools/source/coremltools.converters.mil.input_types.html#enumeratedshapes) to allow several possible input lengths. This change can be tested by converting manually using [`coremltools`](https://github.com/apple/coremltools), and then adapting the code in [`exporters`](https://github.com/huggingface/exporters).
- Update the generation code to select the shortest sequence length from the enumerated set that is larger than the current sequence.