[ci][utils.sh] Look on pytorch servers when installing pip deps

When installing the executorch pip package for CI jobs, look on the
pytorch servers when resolving dependencies. This lets the executorch
package depend on pytorch pre-release and nightly versions.

Author: dbort

.ci/scripts/utils.sh

@@ -18,8 +18,11 @@ retry () {
 install_executorch() {
   which pip
   # Install executorch, this assumes that Executorch is checked out in the
-  # current directory
-  pip install . --no-build-isolation -v
+  # current directory. The --extra-index-url options tell pip to look on the
+  # pytorch servers for nightly and pre-release versions of torch packages.
+  pip install . --no-build-isolation -v \
+      --extra-index-url https://download.pytorch.org/whl/test/cpu \
+      --extra-index-url https://download.pytorch.org/whl/nightly/cpu
   # Just print out the list of packages for debugging
   pip list
 }