update cuda version in unittest_windows_gpu (#4475)

* update cuda ver in win cuda unittest

* add -c condaforge to download torch+cu111

* update comments
Co-authored-by: Prabhat Roy <prabhatroy@fb.com>

.circleci/config.yml

@@ -739,7 +739,7 @@ jobs:
     executor:
       name: windows-gpu
     environment:
-      CUDA_VERSION: "10.2"
+      CUDA_VERSION: "11.1"
       PYTHON_VERSION: << parameters.python_version >>
     steps:
       - checkout

.circleci/config.yml.in

@@ -739,7 +739,7 @@ jobs:
     executor:
       name: windows-gpu
     environment:
-      CUDA_VERSION: "10.2"
+      CUDA_VERSION: "11.1"
       PYTHON_VERSION: << parameters.python_version >>
     steps:
       - checkout

.circleci/unittest/windows/scripts/install.sh

@@ -26,7 +26,8 @@ else
 fi
 
 printf "Installing PyTorch with %s\n" "${cudatoolkit}"
-conda install -y -c "pytorch-${UPLOAD_CHANNEL}" "pytorch-${UPLOAD_CHANNEL}"::pytorch "${cudatoolkit}" pytest
+# conda-forge channel is required for cudatoolkit 11.1 on Windows, see https://github.com/pytorch/vision/issues/4458
+conda install -y -c "pytorch-${UPLOAD_CHANNEL}" -c conda-forge "pytorch-${UPLOAD_CHANNEL}"::pytorch "${cudatoolkit}" pytest
 
 if [ $PYTHON_VERSION == "3.6" ]; then
     printf "Installing minimal PILLOW version\n"