Add torchvision to doc requirements (#141)

Summary:
This is needed by our main example, which otherwise wouldn't properly plot the images

Pull Request resolved: #141

Reviewed By: scotts, ahmadsharif1

Differential Revision: D60596300

Pulled By: NicolasHug

fbshipit-source-id: 1befb6b0a44c9b683ed752ee8a44b403a4ee4663

docs/requirements.txt

@@ -4,4 +4,5 @@ sphinx_design
 sphinx_copybutton
 sphinx-tabs
 matplotlib
+torchvision
 -e git+https://github.com/pytorch/pytorch_sphinx_theme.git#egg=pytorch_sphinx_theme