Fix issue with scalars and __rpow__ (#16687)
Summary:
Changelog:
- Modify __rpow__ function in tensor.py to adapt to scalars
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16687
Differential Revision: D13936720
Pulled By: soumith
fbshipit-source-id: b0c8727968b04efbc6e7461807c812d962f03370
