Fix test_reductions when no SciPy is installed (#61699)
Summary:
Also, use skipIfNoSciPy decorator instead of implicit `unittest.skipIf`
This fixes regression introduced by https://github.com/pytorch/pytorch/pull/52565
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61699
Reviewed By: seemethere
Differential Revision: D29706938
Pulled By: malfet
fbshipit-source-id: 0b63c3ddadfa7f68bed994b71cadf68976d3b396
