Add the reasoning (VLM text-generation) capability. The framework reasoner uses
only the und (causal) pathway weights (no _moe_gen) + embed_tokens / norm /
lm_head — all already present in the native DiT — so a text-only forward + lm_head
reproduces it with no new model code.
- `cosmos3_generate_reasoner_text(transformer, input_ids, max_new_tokens)`:
greedy text reasoning via the und causal backbone + lm_head (text-only prefill,
re-prefill per step; KV-cache fast path is a later optimization). Mirrors the
framework `generate_reasoner_text` (text-only path).
- Parity (`test_cosmos3_reasoning_parity.py`, framework = oracle):
* prefill logits (native text-only forward + lm_head vs framework
`reasoner_forward` + lm_head, per-position): max abs diff = 0.0, mean = 0.0;
* greedy generation vs framework `generate_reasoner_text(do_sample=False)`:
token-for-token identical across seeds.
Text reasoning is fully supported + bit-exact. Image-conditioned reasoning
additionally needs the Qwen3-VL `vision_encoder` (a transformers ViT, tracked
separately). Full suite: 146 passed, 0 skipped.
SolitaryThinker
c8e4034f0a