fix: map yes/no synonyms to their probabilities and normalize

[ChenZiHong-Gavin]

This PR replaces the old yes/no entropy loss with a synonym-aware version:

1. Adds exhaustive English + Chinese synonym lists for “yes” and “no”.
2. Introduces _normalize_yes_no() that pools probabilities of all synonyms and re-normalizes them to a clean {yes: p, no: 1-p} distribution.
3. Updates yes_no_loss_entropy() to use this normalized distribution instead of looking only at the top token, making the loss robust to wording variations.

[gemini-code-assist]

Important

Installation incomplete: to start using Gemini Code Assist, please ask the organization owner(s) to visit the Gemini Code Assist Admin Console and sign the Terms of Services.