Add Nemotron Labs Diffusion model

[Blaizzy]

Summary

• Add the Nemotron Labs Diffusion text model implementation with AR, diffusion, and linear self-speculative generation support.
• Add prompt formatting support and generic --gen-kwargs plumbing for model-specific generation options.
• Add focused diffusion model tests and a model README with usage examples.

Validation

• uv run python -m compileall -q mlx_vlm/models/nemotron_labs_diffusion mlx_vlm/generate/dispatch.py mlx_vlm/utils.py
• uv run python -m pytest mlx_vlm/tests/test_diffusion_models.py mlx_vlm/tests/test_prompt_utils.py -q

[Blaizzy]

To fix

[Blaizzy]

To fix