[Safetensors] Add explicit  flag to from pretrained
## Give user more control when loading safetensors

It would be very helpful if we could give the user more control over loading safetensors checkpoints by adding a `use_safetensors` flag to `from_pretrained`. 

At the moment, `safetensors` weights are **always** loaded when `safetensors` is installed and are **silently** not loaded if `safetensors` is installed, **but** the model has no `safetensors` weights. 

By giving the user the option to do `use_safetensors=True/False` we enable two new use cases:

1.) If a user only wants to load models from safetensors checkpoints, they can now pass `use_safetensors=True` which will lead to an error if no safetensors checkpoints are available => this can then e.g. give the user the guarantee that no pickle is used when loading checkpoints

2.) If a user doesn't want to load `safetensors` checkpoints, but not uninstall the library, they can now pass `use_safetensors=False` which will never load safetensors checkpoitns. This is super helpful for testing as well.

Also this feature would unblock this PR: in `diffusers`. https://github.com/huggingface/diffusers/pull/2123
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