Support PyTorch 2.9

[cdeil]

PyTorch 2.9 shipped with many nice improvements:
https://github.com/pytorch/pytorch/releases/tag/v2.9.0

Could you please make coremltools compatible with it?

[reneleonhardt]

I guess now that PyTorch 2.7 (2025-04-23) support has been released, the first step would be to support PyTorch 2.8 (2025-08-08).
https://github.com/pytorch/pytorch/releases/tag/v2.8.0

Each release introduces many breaking changes, it's unfortunate that there are no prereleases to catch up quicker.