[worfklow] added coverage test (#2399)

* [worfklow] added coverage test

* polish code

* polish code

* polish code

* polish code

* polish code

* polish code

* polish code

* polish code

.github/workflows/build.yml

@@ -20,6 +20,7 @@ jobs:
       - uses: actions/checkout@v2
         with:
           fetch-depth: 0
+          ref: ${{ github.event.pull_request.head.sha }}
       - name: Find the changed files
         id: find-changed-files
         uses: tj-actions/changed-files@v35
@@ -75,7 +76,7 @@ jobs:
 
       - name: Unit Testing
         run: |
-          PYTHONPATH=$PWD pytest tests
+          PYTHONPATH=$PWD pytest --cov=. --cov-report lcov tests
         env:
           DATA: /data/scratch/cifar-10
           NCCL_SHM_DISABLE: 1