Improve error message for Pad operator (#39651)
Summary:
In issue https://github.com/pytorch/pytorch/issues/36997 the user encountered a non-meaningful error message when trying to export the model to ONNX. The Pad operator in opset 9 requires the list of paddings to be constant. This PR tries to improve the error message given to the user when this is not the case.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39651
Reviewed By: hl475
Differential Revision: D21992262
Pulled By: houseroad
fbshipit-source-id: b817111c2a40deba85e4c6cdb874c1713312dba1
