check torch installation before building package. (#1867)

* check cuda installation

* check in build.sh

* use USE_CUDA

* Update pkg_helpers.bash

* Fix typo

packaging/torchaudio/build.sh

 #!/usr/bin/env bash
 set -ex
 
+torch_cuda_version=$(python -c "import torch; print(torch.version.cuda)")
+echo torch.cuda.version is $torch_cuda_version
+
+echo USE_CUDA is "$USE_CUDA"
+
+shopt -s nocasematch
+if [ "${USE_CUDA}" == "1" ] ; then
+    if [ "$torch_cuda_version" == "None" ]; then
+        echo "We want to build torch auido with cuda but the installed pytorch isn't with cuda"
+        exit 1
+    fi
+fi
+shopt -u nocasematch
+
 python setup.py install --single-version-externally-managed --record=record.txt

packaging/torchaudio/meta.yaml

@@ -31,8 +31,8 @@ build:
   string: py{{py}}_{{ environ.get('CU_VERSION', 'cpu') }}
   script_env:
     - BUILD_VERSION
-    - USE_CUDA  # [not win]
-    - TORCH_CUDA_ARCH_LIST  # [not win]
+    - USE_CUDA
+    - TORCH_CUDA_ARCH_LIST
   features:
     {{ environ.get('CONDA_CPUONLY_FEATURE', '') }}