[py-torchvision] added dependency of ffmpeg (#17032)

* [py-torchvision] added dependency of ffmpeg

* [py-torchvision] needs ffmpeg >= 3.1. See: https://github.com/pytorch/vision/issues/1597

* [py-torchvision] ffmpeg dependency is only for py-torchvision versions >= 0.4.2.

var/spack/repos/builtin/packages/py-torchvision/package.py

@@ -42,3 +42,5 @@ class PyTorchvision(PythonPackage):
     # Many of the datasets require additional dependencies to use.
     # These can be installed after the fact.
     depends_on('py-scipy', type='test')
+
+    depends_on('ffmpeg@3.1:', when='@0.4.2:')