Fix gemma-trainer docs and dataset utilities

[houyongsheng]

Summary

• align Gemma SFT examples and distilled records with the standard assistant role
• fix Hugging Face distillation quantization and temperature handling
• let dataset validation fall back to a character-count heuristic when no tokenizer is available
• update quantization, context-window, reward-model, and deployment guidance

Testing

• Python unittest suite: 5 passed
• AST parsing: all 5 gemma-trainer asset scripts passed
• CLI help smoke tests for dataset preparation and distillation

No model weights were downloaded and no GPU training run was performed.