Allow overwriting kernels (#19777)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19777
When used from iPython/Jupyter/Bento, the cell doing kernel registration might be executed multiple times.
Also, there might be a kernel library that wants to overwrite one of the existing kernels.
Let's allow this.
Reviewed By: dzhulgakov
Differential Revision: D15090318
fbshipit-source-id: 09f842e8fd36646053c5c2f11325de4d31105b0c
