The Installation of torch-scatter fails if torch is not already installed

[ThomasCassimon]

When torch-scatter is installed in a system that doesn't have an existing install of torch, the installation fails.

I've noticed that the install_requires in setup.py doesn't list torch as an installation dependency. Adding the torch package here may resolve the issue.

The issue can be easily reproduced by issuing the following commands in a Linux shell:

# First, create a new virtual environment
python3 -m venv test-venv

# Next, activate it
source ~/test-venv/bin/activate

# Finally, try to install `torch-scatter`
python3 -m pip install --upgrade torch-scatter
Looking in indexes: https://pypi.org/simple
Collecting torch-scatter
  Downloading torch_scatter-2.1.1.tar.gz (107 kB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 107.6/107.6 KB 2.0 MB/s eta 0:00:00
  Preparing metadata (setup.py) ... error
  error: subprocess-exited-with-error
  
  × python setup.py egg_info did not run successfully.
  │ exit code: 1
  ╰─> [6 lines of output]
      Traceback (most recent call last):
        File "<string>", line 2, in <module>
        File "<pip-setuptools-caller>", line 34, in <module>
        File "/tmp/pip-install-nksi8_2c/torch-scatter_b525def955bd4685856db2313510ac1d/setup.py", line 8, in <module>
          import torch
      ModuleNotFoundError: No module named 'torch'
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

× Encountered error while generating package metadata.
╰─> See above for output.

note: This is an issue with the package mentioned above, not pip.
hint: See above for details.

The current workaround is to manually install torch first, in a separate command before installing anything that requires torch-scatter.

[ThomasCassimon]

It seems this issue was already reported earlier (#265), closing for now.