message: Update default NCCL version while installing from source to 2.2
  This is to keep the default configuration consistent with prebuilt TensorFlow.
diff: @@ -35,7 +35,7 @@ except ImportError:
  _DEFAULT_CUDA_VERSION = '9.0'
  _DEFAULT_CUDNN_VERSION = '7'
  -_DEFAULT_NCCL_VERSION = '1.3'
  +_DEFAULT_NCCL_VERSION = '2.2'
  _DEFAULT_CUDA_COMPUTE_CAPABILITIES = '3.5,5.2'
  _DEFAULT_CUDA_PATH = '/usr/local/cuda'
  _DEFAULT_CUDA_PATH_LINUX = '/opt/cuda'
  @@ -1097,8 +1097,10 @@ def set_tf_nccl_install_path(environ_cp):
  raise ValueError('Currently NCCL is only supported on Linux platforms.')
  ask_nccl_version = (
  - 'Please specify the NCCL version you want to use. '
  - '[Leave empty to default to NCCL %s]: ') % _DEFAULT_NCCL_VERSION
  + 'Please specify the NCCL version you want to use. If NCCL %s is not '
  + 'installed, then you can use version 1.3 that can be fetched '
  + 'automatically but it may have worse performance with multiple GPUs. '
  + '[Default is %s]: ') % (_DEFAULT_NCCL_VERSION, _DEFAULT_NCCL_VERSION)
  for _ in range(_DEFAULT_PROMPT_ASK_ATTEMPTS):
  tf_nccl_version = get_from_env_or_user_or_default(
  