Autograd for segment_matmul

[puririshi98]

No description provided.

[rusty1s]

I think this perfect. A test would be great to ensure correctness.

[puririshi98]

2: Test command: /home/runner/work/pyg-lib/pyg-lib/build/test_biased_random "--gtest_filter=BiasedSamplingAliasRandomTest.BasicAssertions" "--gtest_also_run_disabled_tests"
2: Test timeout computed to be: 10000000
2: Running main() from /home/runner/work/pyg-lib/pyg-lib/build/_deps/googletest-src/googletest/src/gtest_main.cc
2: Note: Google Test filter = BiasedSamplingAliasRandomTest.BasicAssertions
2: [==========] Running 1 test from 1 test suite.
2: [----------] Global test environment set-up.
2: [----------] 1 test from BiasedSamplingAliasRandomTest
2: [ RUN      ] BiasedSamplingAliasRandomTest.BasicAssertions
2: /home/runner/work/pyg-lib/pyg-lib/test/csrc/random/test_biased_random.cpp:74: Failure
2: Expected: (cnt[1]) < (0.7 * iter), actual: 701 vs 700
2: [  FAILED  ] BiasedSamplingAliasRandomTest.BasicAssertions (1 ms)
2: [----------] 1 test from BiasedSamplingAliasRandomTest (1 ms total)
2: 
2: [----------] Global test environment tear-down
2: [==========] 1 test from 1 test suite ran. (1 ms total)
2: [  PASSED  ] 0 tests.
2: [  FAILED  ] 1 test, listed below:
2: [  FAILED  ] BiasedSamplingAliasRandomTest.BasicAssertions
2: 
2:  1 FAILED TEST

the last CI failure seems unrelated to my changes.

[puririshi98]

also unsure how to integrate the python test I wrote as part of this PR into the CI for this project

[codecov-commenter]

Codecov Report

Merging #61 (28d735a) into master (5b48534) will decrease coverage by 6.79%.
The diff coverage is 0.00%.

❗ Current head 28d735a differs from pull request most recent head c0a534f. Consider uploading reports for the commit c0a534f to get more accurate results

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   90.68%   83.89%   -6.80%     
==========================================
  Files          13       13              
  Lines         247      267      +20     
==========================================
  Hits          224      224              
- Misses         23       43      +20     

Impacted Files	Coverage Δ
pyg_lib/csrc/ops/matmul.cpp	11.76% <0.00%> (-16.81%)	⬇️

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[rusty1s]

Thanks @puririshi98. I made some further changes. Also testing autograd on the CPP-level now. I am in favor of merging this but ultimately there is a backward pass missing to compute other_grad. I don't think we can implement this via segment_matmul - it probably needs its own implementation. Let's sync later on this.