[ONNX] Add symbolic for _convolution_mode (#89107)
As per #68880
implement the operator _convolution_mode in the ONNX exporter. This will allow user to leverage the padding 'str' mode where it can be set to 'valid' or 'same'.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89107
Approved by: https://github.com/titaiwangms, https://github.com/BowenBao
