Add minimal [segment|grouped]_matmul CPU implementation

[DamianSzwichtenberg]

Added minimal [segment|grouped]_matmul CPU implementation, using at::matmul_out.

Next steps (other PR):
Optimize code using gemm_batched (preview).

Update:
An additional fix was provided for grouped_matmul which occurred on both CPU and GPU.

C++ exception with description "There were no tensor arguments to this function (e.g., you passed an empty list of Tensors), but no fallback function is registered for schema pyg::grouped_matmul.  This usually means that this function requires a non-empty list of Tensors, or that you (the operator writer) forgot to register a fallback function.

It was caused by the wrong grouped_matmul function signature - it was not aligned with the schema provided at TORCH_LIBRARY_FRAGMENT call. As stated here, Tensor[] as input is equivalent of at::TensorList.

[codecov-commenter]

Codecov Report

Merging #111 (7cadd0a) into master (285bd21) will increase coverage by 5.10%.
The diff coverage is 100.00%.

❗ Current head 7cadd0a differs from pull request most recent head a642ab8. Consider uploading reports for the commit a642ab8 to get more accurate results

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
+ Coverage   89.23%   94.33%   +5.10%     
==========================================
  Files          16       19       +3     
  Lines         418      477      +59     
==========================================
+ Hits          373      450      +77     
+ Misses         45       27      -18     

Impacted Files	Coverage Δ
pyg_lib/csrc/ops/matmul.h	100.00% <ø> (ø)
pyg_lib/csrc/ops/cpu/matmul_kernel.cpp	100.00% <100.00%> (ø)
pyg_lib/csrc/ops/matmul.cpp	91.83% <100.00%> (+73.65%)	⬆️
pyg_lib/csrc/utils/check.cpp	100.00% <100.00%> (ø)
pyg_lib/csrc/sampler/neighbor.cpp	100.00% <0.00%> (ø)
pyg_lib/csrc/sampler/cpu/neighbor_kernel.cpp	89.71% <0.00%> (+0.33%)	⬆️
pyg_lib/csrc/utils/convert.cpp	100.00% <0.00%> (+100.00%)	⬆️

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[DamianSzwichtenberg]

@rusty1s It seems that docs build is failing because of:

File "/home/docs/checkouts/readthedocs.org/user_builds/pyg-lib/envs/111/lib/python3.8/site-packages/sphinx_rtd_theme/layout.html", line 13, in top-level template code
    {%- set (_ver_major, _ver_minor, _ver_bugfix) = sphinx_version.split('.') | map('int') -%}
ValueError: too many values to unpack (expected 3)

The installed version is now 5.2.0.post0... Maybe we should specify the version explicitly in requirements.txt?

[rusty1s]

Yes, will look into.

[rusty1s]

Seems like sphinx==5.2.1 fixed this issue.

[DamianSzwichtenberg]

@rusty1s Please take a look. 😉

[rusty1s]

Thanks for this PR and the dispatcher fix. I really had no idea how one could resolve it :)