Temporarily pin nightly version on Linux/macOS CPU unittest (#1598)

There are some issues on CMake-based integration after 2021-06-19.

```
  Imported target "torch" includes non-existent path
    "/opt/conda/conda-bld/pytorch_1624259172741/work/torch/lib"
  in its INTERFACE_INCLUDE_DIRECTORIES.  Possible reasons include:
  * The path was deleted, renamed, or moved to another location.
  * An install or uninstall procedure did not complete successfully.
  * The installation package was faulty and references files it does not provide.
```

This is being worked on PR like https://github.com/pytorch/pytorch/pull/60403.
The nightly builds are still causing the error, so as a workaround, 
we pin the nightly build version for Linux/macOS unittest.
The issue still happens on Windows unittest and binary builds.

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

@@ -23,21 +23,41 @@ eval "$("${conda_dir}/bin/conda" shell.bash hook)"
 conda activate "${env_dir}"
 
 # 1. Install PyTorch
-if [ -z "${CUDA_VERSION:-}" ] ; then
-    if [ "${os}" == MacOSX ] ; then
-        cudatoolkit=''
-    else
-        cudatoolkit="cpuonly"
-    fi
+# [2021/06/22 Temporary workaround] Disabling the original installation
+# The orignal, conda-based instartion is working for GPUs, but not for CPUs
+# For CPUs we use pip-based installation
+# if [ -z "${CUDA_VERSION:-}" ] ; then
+#     if [ "${os}" == MacOSX ] ; then
+#         cudatoolkit=''
+#     else
+#         cudatoolkit="cpuonly"
+#     fi
+# else
+#     version="$(python -c "print('.'.join(\"${CUDA_VERSION}\".split('.')[:2]))")"
+#     cudatoolkit="cudatoolkit=${version}"
+# fi
+# printf "Installing PyTorch with %s\n" "${cudatoolkit}"
+# (
+#     set -x
+#     conda install ${CONDA_CHANNEL_FLAGS:-} -y -c "pytorch-${UPLOAD_CHANNEL}" "pytorch-${UPLOAD_CHANNEL}::pytorch" ${cudatoolkit}
+# )
+
+if [ "${os}" == MacOSX ] || [ -z "${CUDA_VERSION:-}" ] ; then
+    device="cpu"
+    printf "Installing PyTorch with %s\n" "$device}"
+    (
+        set -x
+        pip install --pre torch==1.10.0.dev20210618 -f "https://download.pytorch.org/whl/nightly/${device}/torch_nightly.html"
+    )
 else
     version="$(python -c "print('.'.join(\"${CUDA_VERSION}\".split('.')[:2]))")"
     cudatoolkit="cudatoolkit=${version}"
-fi
-printf "Installing PyTorch with %s\n" "${cudatoolkit}"
-(
+    printf "Installing PyTorch with %s\n" "${cudatoolkit}"
+    (
         set -x
         conda install ${CONDA_CHANNEL_FLAGS:-} -y -c "pytorch-${UPLOAD_CHANNEL}" "pytorch-${UPLOAD_CHANNEL}::pytorch" ${cudatoolkit}
-)
+    )
+fi
 
 # 2. Install torchaudio
 printf "* Installing torchaudio\n"