Odds ratio: rescore saved states and expose per-step divergence traces

- `_calculate_odds_ratio_full(start_latents=...)`: skip the forward phase and
  run the two backward phases on a given state, so saved samples can be
  rescored under different sampler settings without regenerating them.
- `OddsRatioResult.{forward,backward}_divergence_data`: optional per-step
  DivergenceTracker records (sigma, divergence, score_divergence).
- `scripts/inference_odds_ratio.py`: batch driver for the TC odds-ratio runs,
  with CLI flags for num steps, extra steps / window, guidance window,
  Hutchinson samples, `--x0-dir` rescoring and `--save-divergence-traces`.

Public signatures of `sample()` and `calculate_odds_ratio()` are unchanged.

Signed-off-by: Peter Manshausen <pmanshausen@nvidia.com>

Author: Peter Manshausen