* Fix Qwen3 NaN: self-heal vision pad_token in load_correct_tokenizer
Text-only Qwen3 (and Qwen2.5) models share Qwen3-VL's vocab, so their
Hub tokenizer configs ship <|vision_pad|> as pad_token. Padding text-only
training with a vision token corrupts attention/loss and produces NaN
losses and gradients on affected stacks.
patch_tokenizer already heals this, but only when a model with config is
passed. The standalone load_correct_tokenizer path (and custom training
loops) still returned <|vision_pad|>. This adds a model-independent guard
in load_correct_tokenizer that replaces a vision pad_token on text-only
tokenizers with the first safe text token (<|endoftext|>, <pad>, [PAD],
<unk>), falling back to eos_token only if it differs from pad_token.
The result now matches upstream Qwen configs (pad_token <|endoftext|>,
id 151643) with no new token added. Vision processors (image_processor
present) and non-vision pad tokens (Llama, Qwen2) are left untouched.
Fixes#3155
* Format pad_token helper for ruff kwarg-spacing hook (pre-commit)
* Harden vision pad_token fix: drop unk candidate, guard get_vocab, skip vision eos
* Tighten code comments (no logic change)
* Delegate pad_token fix to shared unsloth_zoo.pad_token
Generalize the narrow _fix_vision_pad_token by delegating to unsloth_zoo's
shared fix_pad_token (single source of truth, AGPL-3.0), which scans the
reserved-token families instead of only the vision-pad case. A guarded import
keeps this working against an older unsloth_zoo that has not shipped the module
yet: on ImportError it falls back to _fix_vision_pad_token.
allow_add=False is passed so the early load_correct_tokenizer call stays
side-effect free (no model here to resize embeddings); the later model-aware
patch_tokenizer call finishes the job and is idempotent.
Adds tests/python/test_pad_token_fix.py covering both dispatch paths offline.
b91cdc8793