[CI] Fix windows CI (#954)

Author: vmoens

.circleci/unittest/windows_optdepts/scripts/install.sh

@@ -38,7 +38,11 @@ fi
 git submodule sync && git submodule update --init --recursive
 
 printf "Installing PyTorch with %s\n" "${cudatoolkit}"
-conda install -y -c "pytorch-${UPLOAD_CHANNEL}" -c nvidia pytorch "${cudatoolkit}"
+if $torch_cuda ; then
+  pip3 install --pre torch torchvision torchaudio --index-url https://download.pytorch.org/whl/nightly/cu118
+else
+  pip3 install --pre torch torchvision torchaudio --index-url https://download.pytorch.org/whl/nightly/cpu
+fi
 
 torch_cuda=$(python -c "import torch; print(torch.cuda.is_available())")
 echo torch.cuda.is_available is $torch_cuda

.circleci/unittest/windows_optdepts/scripts/setup_env.sh

@@ -37,3 +37,6 @@ conda activate "${env_dir}"
 # 3. Install Conda dependencies
 printf "* Installing dependencies (except PyTorch)\n"
 conda env update --file "${this_dir}/environment.yml" --prune
+
+# we don't use torchsnapshot
+conda env config vars set CKPT_BACKEND=torch