Change release_type on release branch for testing workflow (#717)

Summary:
When `input_branch` is empty, the testing workflow should determine which PyTorch release type should be used for testing PR.

You can find the testing result from the PR #716

Pull Request resolved: #717

Reviewed By: NivekT

Differential Revision: D38436132

Pulled By: ejguan

fbshipit-source-id: bfa9a5e2b23d03b378efdd4b0f996b32d6870e3a

Author: ejguan

.github/workflows/_build_test_upload.yml

@@ -43,7 +43,11 @@ jobs:
           elif [[ "${{ inputs.branch }}" == release/* ]] && [[ ${{ inputs.pre_dev_release }} == true ]]; then
             RELEASE_TYPE=test
           else
-            RELEASE_TYPE=nightly
+            if [[ "${{ github.base_ref }}" == release/* ]]; then
+              RELEASE_TYPE=test
+            else
+              RELEASE_TYPE=nightly
+            fi
           fi
           echo "Release Type: $RELEASE_TYPE"
           echo "::set-output name=type::$RELEASE_TYPE"