Wire text-to-video+sound end-to-end, completing the audio PR.
- CFG velocity (`cosmos3_get_cfg_velocity`) + `Cosmos3DenoiseEngine` take an
optional `sound_specs`: the denoise latent becomes a combined [vision | sound]
flat vector; each step packs vision+sound, forwards the DiT, zeros the
prediction on conditioning frames for both, and returns the concatenated
velocity. The UniPC scheduler steps the joint vector. `Cosmos3SoundSpec` +
`_split_sound_latent` added.
- `Cosmos3DenoisingStage` t2vs path (gated on `COSMOS3_T2VS`): size the sound
latent from the video duration (framework `create_placeholder_audio` +
`get_latent_num_samples`), append sound noise, joint-denoise, split, AVAE-decode
the sound latent, and set `batch.extra["audio"]` / `["audio_sample_rate"]` so
the generator muxes a stereo 48 kHz AAC track into the mp4. Sound AVAE is
lazy-loaded from `<model_path>/sound_tokenizer` (`_get_sound_vae`).
- Parity (`test_cosmos3_sound_parity::test_t2vs_cfg_velocity_matches_framework`):
the combined [vision|sound] sequential-CFG velocity vs a framework-DiT oracle —
max abs diff = 0.0, mean abs diff = 0.0 across t2vs/i2vs cases.
- Example `basic_cosmos3_t2vs_new_api.py`.
Verified on B200 (real weights, 1280x704, 29f, 35 steps): coherent ocean/sunset
video WITH a real stereo 48 kHz audio track (mean -10.2 dB, peak -0.4 dB; not
silence). Full cosmos3 suite: 130 passed, 0 skipped.
SolitaryThinker
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