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Error when building in Docker container:
Found no NVIDIA driver on your system. Please check that you
have an NVIDIA GPU and installed a driver from
http://www.nvidia.com/Download/index.aspx

Collecting torch-scatter
  Downloading https://files.pythonhosted.org/packages/b8/c3/8bad887ffa55c86f120ef5ae252dc0e357b3bd956d9fbf45242bacc46290/torch_scatter-1.4.0.tar.gz
Building wheels for collected packages: torch-scatter
  Building wheel for torch-scatter (setup.py): started
  Building wheel for torch-scatter (setup.py): finished with status 'error'
  ERROR: Command errored out with exit status 1:
   command: /usr/bin/python3 -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-xenlbljt/torch-scatter/setup.py'"'"'; __file__='"'"'/tmp/pip-install-xenlbljt/torch-scatter/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))...

Implement scatter_logsumexp, scatter_softmax, scatter_log_softmax

* `log_softmax` has now stand-alone to save one operation (and fix a bug).
* `softmax` is implemented in a similar stand-alone way.
* Address some PR comments.