[Proposal] Support saving to safetensors (#1494)
* Add parameter safe_serialization to DiffusionPipeline.save_pretrained
* Add option safe_serialization on ModelMixin.save_pretrained
* Add test test_save_safe_serialization
* Black
* Re-trigger the CI
* Fix doc-builder
* Validate files are saved as safetensor in test_save_safe_serialization
