Add support to save safetensors checkpoint directly into onnx (#121001)
Currently, when `torch.onnx.dynamo_export` is called within `torch.onnx.enable_fake_mode`, all the external pytorch checkpoint files used to initialize the model are automatically and used by `torch.onnx.ONNXProgram.save` to recreate the initializers for
the newly exported ONNX model.
This API extends the mechanism for HuggingFace models that use safetensors weights. This PR detects safetensors state files and converts them to PyTorch format using mmap on a temporary file, which is deleted after conversion is finished.
Without this PR, the user would have to convert the safetensors files to pytorch format manually and feed it to `torch.onnx.ONNXProgram.save` manually.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121001
Approved by: https://github.com/BowenBao, https://github.com/malfet
