Use safetensors by default for `PyTorchModelHubMixin` (#2033)
* Use safetensors in `PyTorchModelHubMixin`
* Use safetensors constant
* Fix default download location and add test
* Fallback to pickle model
* make style
* Load safetensors dynamically
* Migrate to new API
* Add types
* improve assertion for safetensor header
* Use @patch without the delegate
* make style
* patch hf_hub_download
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Co-authored-by: Lucain Pouget <lucainp@gmail.com>