Add scipy as a dependency to setup.py (#675)

* Add scipy as a dependency to setup.py

`scipy` is imported in both the `torchvision.datasets.svhn` module and the `torchvision.models.inception` module.

* Update setup.py

Make scipy optional. It can be installed with `pip install torchvision[scipy]`.

setup.py

@@ -64,4 +64,7 @@ setup(
 
     zip_safe=True,
     install_requires=requirements,
+    extras_require={
+        "scipy": ["scipy"],
+    },
 )