feature: Add Keras 3 support for multi-backend (pytorch, jax)

[rmarquis]

Feature request

Allows bentoml.keras to work with Keras models using the PyTorch backend without requiring TensorFlow.

Motivation

Keras 3 is now multi-backend. It should be allowed to use PyTorch as backend instead of TensorFlow.

Other

No response

[MoeenAsim]

Hi, I'd like to work on this issue.

[rmarquis]

There's already a PR that implements the required changes.