Remove RTD theme and replace with pytorch_sphinx_theme

Author: ericnakagawa

.jenkins/build.sh

@@ -21,7 +21,7 @@ export PATH=/opt/conda/bin:$PATH
 # pillow >= 4.2 will throw error when trying to write mode RGBA as JPEG,
 # this is a workaround to the issue.
 conda install -y sphinx pandas pillow=4.1.1
-pip install sphinx-gallery sphinx_rtd_theme tqdm matplotlib ipython
+pip install sphinx-gallery pytorch_sphinx_theme tqdm matplotlib ipython
 
 git clone https://github.com/pytorch/vision --quiet
 pushd vision