Strip doc_string from exported ONNX models (#18882)

Strip doc_string from exported ONNX models (#18882)

Summary:
Strip the doc_string by default from the exported ONNX models (this string has the stack trace and information about the local repos and folders, which can be confidential).

The users can still generate the doc_string by specifying add_doc_string=True in torch.onnx.export().
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18882

Differential Revision: D14889684

Pulled By: houseroad

fbshipit-source-id: 26d2c23c8dc3f484544aa854b507ada429adb9b8