Add safetensors to model not found error msg for default use_safetensors value
Author: davidgxue
Pull request: yes

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Fixes #30601

- See details in the issue linked
- TL;DR: Most people load using `.from_pretrained()` without explicitly specifying `use_safetensors` arg. This `use_safetensors` arg will default to `None` if not specified AND if system has `safetensors` library installed. When a model is being loaded and no modeling file is found and `use_safetensors` is defauled to `None`, `model.safetensors` does not get mentioned as part of the error message that lists out all the model files formats it has attempted to load, leading to confusion that `safetensors` may not be supported or should not be used.
- There are error messages specifically made for loading `safetensors` but they only cover the explicitly `True` case. The idea here is that the comprehensive error message should list all model types when `use_safetensors` is defaulted to `None`.


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