Use safetensors by default for `PyTorchModelHubMixin` #5511

Job	Run time
build-windows (3.8)	12m 57s
build-ubuntu (3.8, Repository only)	2m 8s
build-ubuntu (3.8, Everything else)	8m 16s
build-ubuntu (3.8, torch)	2m 22s
build-ubuntu (3.11, Repository only)	1m 57s
build-ubuntu (3.11, Everything else)	5m 30s
build-ubuntu (3.11, torch)	2m 12s
build-ubuntu (3.11, lfs)	1m 22s
build-ubuntu (3.8, fastai)	2m 35s
build-ubuntu (3.10, fastai)	2m 35s
build-ubuntu (3.8, tensorflow)	2m 2s
build-ubuntu (3.10, tensorflow)	2m 6s
	46m 2s

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