`test` should not have `__init__.py`

[ZenoTan]

Listing my site-packages, ls /home/zy/miniconda3/envs/zy/lib/python3.9/site-packages/test/, will show:

__init__.py           test_coalesce.py    test_gather.py           test_overload.py  test_segment.py  test_transpose.py
__pycache__           test_convert.py     test_matmul.py           test_permute.py   test_spmm.py     test_zero_tensors.py
test_add.py           test_diag.py        test_metis.py            test_saint.py     test_spspmm.py   utils.py
test_broadcasting.py  test_ego_sample.py  test_multi_gpu.py        test_sample.py    test_storage.py
test_cat.py           test_eye.py         test_neighbor_sample.py  test_scatter.py   test_tensor.py

After a short investigation, the reason might be we have __init__.py under test and it is installed as a package.

Another way to show:

pip uninstall torch_sparse
Found existing installation: torch-sparse 0.6.15
Uninstalling torch-sparse-0.6.15:
  Would remove:
    /home/zy/miniconda3/envs/zy/lib/python3.9/site-packages/test/*
    /home/zy/miniconda3/envs/zy/lib/python3.9/site-packages/torch_sparse-0.6.15.dist-info/*
    /home/zy/miniconda3/envs/zy/lib/python3.9/site-packages/torch_sparse/*
  Would not remove (might be manually added):
    /home/zy/miniconda3/envs/zy/lib/python3.9/site-packages/test/test_broadcasting.py
    /home/zy/miniconda3/envs/zy/lib/python3.9/site-packages/test/test_gather.py
    /home/zy/miniconda3/envs/zy/lib/python3.9/site-packages/test/test_multi_gpu.py
    /home/zy/miniconda3/envs/zy/lib/python3.9/site-packages/test/test_scatter.py
    /home/zy/miniconda3/envs/zy/lib/python3.9/site-packages/test/test_segment.py
    /home/zy/miniconda3/envs/zy/lib/python3.9/site-packages/test/test_zero_tensors.py

[rusty1s]

This is indeed an anti-pattern :( Will be fixed in #298.