- Add no-op shim for torch.utils._pytree.register_constant in train.py
before any torchao/unsloth/transformers import. Fixes AttributeError
when torchao>=0.9 is installed with PyTorch 2.6.0.
- Dockerfile: move torchao==0.6.1 force-reinstall to LAST pip step so
nothing can override it after the fact.
- Dockerfile: tighten transformers pin to <4.48 (4.48+ unconditionally
imports quantizer_torchao at module init, breaking torchao 0.6.1).
- Dockerfile: remove --upgrade from 'pip install -r requirements.txt'
to prevent silent package version bumps.
Force-downgrade torchao to 0.6.1 after unsloth to fix register_constant crash
