Introducing Matrix exponential (#40161)
Summary:
Implements (batched) matrix exponential. Fixes [https://github.com/pytorch/pytorch/issues/9983](https://github.com/pytorch/pytorch/issues/9983).
The algorithm follows:
```
Bader, P.; Blanes, S.; Casas, F.
Computing the Matrix Exponential with an Optimized Taylor Polynomial Approximation.
Mathematics 2019, 7, 1174.
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40161
Reviewed By: zhangguanheng66
Differential Revision: D22951372
Pulled By: ezyang
fbshipit-source-id: aa068cb76d5cf71696b333d3e72cee287b3089e3
