Allow discrete sequence lengths (enumerated input shapes)

[pcuenca]

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 to allow several possible input lengths. This change can be tested by converting manually using coremltools, and then adapting the code in exporters.
• Update the generation code to select the shortest sequence length from the enumerated set that is larger than the current sequence.