Avoid space in NVTXHook session hook's range message

[shawnwang18]

When using nsight-system range capture feature together with nvtx-plugins, range name with blank can no be processed normally by nsight-system, so would better avoid space in range message.

related code:
https://github.com/NVIDIA/nvtx-plugins/blob/master/nvtx_plugins/python/nvtx/plugins/tf/estimator.py#L36