Use master branch of pytorch_lightning on github CI

Summary: Pull Request resolved: https://github.com/facebookresearch/d2go/pull/68

Reviewed By: kazhang

Differential Revision: D28426745

Pulled By: zhanghang1989

fbshipit-source-id: 5b82b03847e5e1d06d1bfcdaf0192d65e9202e77

.github/workflows/unit_test.yml

@@ -20,6 +20,7 @@ jobs:
         run: |
           conda install pytorch torchvision -c pytorch-nightly
           python -m pip install 'git+https://github.com/facebookresearch/detectron2.git'
+          python -m pip install 'git+https://github.com/PyTorchLightning/pytorch-lightning.git'
           pip install git+https://github.com/facebookresearch/mobile-vision
           pip install scikit-learn
           pip install pytest