Fix CI by using pip instead of conda to install torch (#863)

Author: zou3519

.circleci/unittest/linux/scripts/install.sh

@@ -58,6 +58,6 @@ if [ "${CU_VERSION:-}" == cpu ] ; then
     pip install torch torchvision -f https://download.pytorch.org/whl/nightly/cpu/torch_nightly.html --pre
     PYTORCH_VERSION="$(python -c "import torch; print(torch.__version__)")" python setup.py develop bdist_wheel -d $WHEELS_FOLDER
 else
-    conda install -y pytorch torchvision cudatoolkit=10.2 -c pytorch-nightly
+    pip install torch torchvision -f https://download.pytorch.org/whl/nightly/cu102/torch_nightly.html --pre
     PYTORCH_VERSION="$(python -c "import torch; print(torch.__version__)")" python setup.py develop bdist_wheel -d $WHEELS_FOLDER
 fi