Add pip cache purge to install_via_pip (meta-pytorch#1300)

Summary:
Pull Request resolved: meta-pytorch#1300

We occasionally hit hash mismatch error in Github tests. Although it usually can be solve with a rerun, it's still a waste of time. Hopefully purge pip cache before install anything can ensure that we won't see this error anymore.

Reviewed By: cyrjano

Differential Revision: D58955033

fbshipit-source-id: 96fb0c1ce60eddb2d2d340328b4efe2f5b86faa4

Author: yucu

scripts/install_via_pip.sh

@@ -34,6 +34,9 @@ sudo apt install yarn
 # yarn needs terminal info
 export TERM=xterm
 
+# Remove all items from pip cache to avoid hash mismatch
+pip cache purge
+
 # upgrade pip
 pip install --upgrade pip --progress-bar off
 
@@ -54,7 +57,7 @@ else
   if [[ $CHOSEN_TORCH_VERSION == -1 ]]; then
     pip install --upgrade torch --progress-bar off
   else
-    pip install torch==$CHOSEN_TORCH_VERSION --progress-bar off
+    pip install torch=="$CHOSEN_TORCH_VERSION" --progress-bar off
   fi
 fi