fix gpu coverage

Author: ChengjieLi28

.github/workflows/core-gpu-ci.yml

@@ -46,7 +46,11 @@ jobs:
       - name: Run GPU CI
         shell: bash
         run: |
+          mkdir build_coverage/
           coverage run -m pytest ./ -m cuda --cov-config=setup.cfg --cov-report= --cov=mars
+          coverage combine
+          mv .coverage build_coverage/.coverage.gpu_ctx.file
+          coverage combine build_coverage/ && coverage report
           coverage xml
 
       - name: Report coverage data
@@ -55,3 +59,4 @@ jobs:
           bash <(curl -s https://codecov.io/bash)
           rm -rf *.coverage*
           rm -rf coverage.xml
+          rm -rf build_coverage/