Add pytorch 1.6 CI (open-mmlab#64)

Author: xvjiarui

.github/workflows/build.yml

@@ -47,6 +47,9 @@ jobs:
           - torch: 1.5.0+cu101
             torchvision: 0.6.0+cu101
             python-version: 3.7
+          - torch: 1.6.0+cu101
+            torchvision: 0.7.0+cu101
+            python-version: 3.7
 
     steps:
       - uses: actions/checkout@v2