Add LTX2 Transformer

[prishajain1]

This PR adds the transformer component for LTX2 Model along with the unittests for the same

New files added:

• transformer_ltx2.py : Transformer component for LTX2
• test_transformer_ltx2.py : unittests for transformer
• ltx2_video.yml : config file for ltx2_video.yml

Files Modified:

• common_types.py, pyconfig.py : for adding LTX2_VIDEO to the list of allowed model names for inference
• embeddings_flax.py
  • Refactored sinusoidal timestep embedding generation into a dedicated nnx.Module called NNXTimesteps for better modularity.
  • A new module NNXPixArtAlphaCombinedTimestepSizeEmbeddings combines timestep embeddings with optional resolution and aspect ratio embeddings

[github-actions]

e2e testgrid: https://8bcf50593faf4ea38060e236169827e5-dot-us-central1.composer.googleusercontent.com/dags/maxdiffusion_tpu_e2e/grid