Add Keras 3 example for "Transformer model for MIDI music generation" (#1992)

* Pre-PR review: base script file working in tf

* Fix too slow inference on jax

* Refactor MIDI generation script: PR feedback.
- Update import statement for keras_hub
- Fix documentation for audio dependencies
- Remove unnecessary markdown annotation

Other changes:
- Update last modified date in MIDI generation example
- Fix: Increase training epochs from 1 to 20 in MIDI generation example
- Fix: use flip for unsupported negative step in torch
- Fix: Remove unused learning rate

* Torch backend fixes: add cast on generate, change learning rate scale

* Compact script

* Add auto generated files

Author: johacks