fix(assignment): fix indexing when matrix is transposed

[gcanat]

Forgot to adapt the indexing when matrix is transposed.
Modified a test to display the issue: without the fix the test fails.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.62%. Comparing base (db09f08) to head (515cd28).
⚠️ Report is 35 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
- Coverage   98.77%   93.62%   -5.16%     
==========================================
  Files          17       15       -2     
  Lines        2703     3419     +716     
==========================================
+ Hits         2670     3201     +531     
- Misses         33      218     +185     

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[codspeed-hq]

Merging this PR will degrade performance by 42.1%

❌ 17 regressed benchmarks
✅ 158 untouched benchmarks
⏩ 1 skipped benchmark¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	test_rtree_nms_many_boxes[5000]	4.6 ms	10.9 ms	-57.52%
❌	test_rtree_nms_many_boxes[20000]	21.3 ms	50.2 ms	-57.46%
❌	test_rtree_nms_many_boxes[10000]	10 ms	23.1 ms	-56.97%
❌	test_rtree_nms_many_boxes[1000]	842.8 µs	1,838.6 µs	-54.16%
❌	test_rtree_rotated_nms_many_boxes[5000]	6.4 ms	12.7 ms	-49.96%
❌	test_rtree_rotated_nms_many_boxes[10000]	13.6 ms	27.1 ms	-49.76%
❌	test_rtree_rotated_nms_many_boxes[1000]	1.1 ms	2.1 ms	-46.49%
❌	test_rtree_nms[float32]	105.9 µs	173.7 µs	-39.06%
❌	test_rtree_nms[float64]	108.6 µs	171.5 µs	-36.71%
❌	test_rtree_nms[int32]	106.6 µs	164.8 µs	-35.29%
❌	test_rtree_nms[int64]	109.4 µs	166.5 µs	-34.31%
❌	test_rtree_nms[int16]	98.8 µs	147 µs	-32.8%
❌	test_rtree_rotated_nms[int16]	137.4 µs	199.8 µs	-31.2%
❌	test_rtree_rotated_nms[int32]	136.7 µs	196.6 µs	-30.46%
❌	test_rtree_rotated_nms[int64]	136.1 µs	194.3 µs	-29.93%
❌	test_rtree_rotated_nms[float32]	136 µs	191.5 µs	-28.96%
❌	test_rtree_rotated_nms[float64]	137.4 µs	186.9 µs	-26.47%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing gcanat:fix/transpose_idx (515cd28) with main (6f1c650)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩