Add LTX-2 19B Dev Model video generation pipeline with Classifier-Free Guidance (CFG)

[Blaizzy]

Summary

• Add LTX-2 19B Dev Model video generation pipeline with Classifier-Free Guidance (CFG)
• Optimize RoPE computation with precomputed frequencies and double precision support
• Add comprehensive test suites for generate_dev, RoPE, and VAE streaming

Test Plan

• Run pytest tests/test_generate_dev.py -v - verify all 32 tests pass
• Test text-to-video generation: python -m mlx_video.generate_dev --prompt "A cat walking" --steps 20
• Test image-to-video generation: python -m mlx_video.generate_dev --prompt "A cat walking" --image input.png

[testacode]

Hey! Looking forward to this merge.

Currently using mlx-video with mlx-community/LTX-2-dev-bf16 and hitting the hardcoded filename issue (ltx-2-19b-distilled.safetensors vs actual ltx-2-19b-dev.safetensors).

This PR would fix it. Any ETA on merge? Happy to help test if needed!

[Blaizzy]

@testacode thanks for the patience!

I have another PR that includes this one, just give me a couple days