Add FLUX support with MLX backend

[vishwamartur]

Related to #461

Add support for the FLUX model with the MLX backend.

• New Model Class: Add exo/inference/mlx/models/flux.py to define the FluxModel and Model classes for the FLUX model.
• Inference Engine Update: Modify exo/inference/mlx/sharded_inference_engine.py to include necessary imports for the FLUX model.
• Testing: Add exo/inference/mlx/test_sharded_flux.py to include tests for the FLUX model, ensuring proper integration and functionality.
• Documentation: Update README.md to include instructions for using the FLUX model with the MLX backend, along with usage examples.

[AlexCheema]

Did you test this?

[AlexCheema]

I don't think this works. Please re-open if that's not that case.