fix: handle list-type _tied_weights_keys in _get_tied_weight_keys

[gh-wf]

Some models (e.g. Nemotron-H) define _tied_weights_keys as a list, which caused AttributeError: 'list' object has no attribute 'keys' when calling save_pretrained during full finetuning.

What does this PR do?

_get_tied_weight_keys in modeling_utils.py calls .keys() on the _tied_weights_keys attribute, assuming it's
always a dict. However, some models (e.g. Nemotron-H) define _tied_weights_keys as a list, causing an
AttributeError when saving checkpoints during full finetuning.

This fix handles both list/tuple and dict types for _tied_weights_keys.

Traceback:
File "transformers/modeling_utils.py", line 341, in _get_tied_weight_keys
tied_weight_keys.extend([f"{name}.{k}" if name else k for k in tied.keys()])
AttributeError: 'list' object has no attribute 'keys'

Reproduction: Full finetune of NVIDIA-Nemotron-3-Nano-4B via Unsloth Studio with Transformers 5.3.0 crashes at
checkpoint save.

Fixes #44861

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AI Disclosure: This PR was developed with assistance from Claude Code (Claude Opus 4.6). All changes were reviewed
and validated by a human.

Who can review?

@Cyrilvallez — model loading / save_pretrained code path