Conversion between SparseBsr and Strided (#78025)
Adds conversion between the strided and SparseBsr layout
[Based on code by @bhosmer!](https://colab.research.google.com/drive/1NHWti04TU269dzbRjLfxGxVlzZWo1XLo?usp=sharing)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78025
Approved by: https://github.com/pearu, https://github.com/jbschlosser
