Fix ConvTranspose with padding as a list of values (#54911)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/54452
The assertion that fails in the issue is necessary to appease mypy. Instead, I fix `_ntuple` to always return a `tuple`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54911
Reviewed By: H-Huang
Differential Revision: D27411088
Pulled By: jbschlosser
fbshipit-source-id: 7f5045c58dd4f5f3b07b4826d9b4ca85606c5bce
