Add failing test for fmod autodiff gradient bug (#10071)

[rkoivunen-sw]

Adds failing test for #10071

The test verifies fmod(x, y) backward-mode autodiff gradient w.r.t. y. Currently returns 0, should return -floor(x/y).

This test will fail CI until the bug is fixed.

Expected output:

type: float
-2.000000  <- d/dy (currently wrong: 0)
1.000000   <- d/dx (correct)
1.500000   <- primal (correct)

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📝 Walkthrough

Walkthrough

A new regression test for backward-mode automatic differentiation of the fmod operation is added to the Slang test suite, consisting of a compute shader test and its expected output file.

Changes

Cohort / File(s)	Summary
Test Implementation tests/autodiff/fmod-backward-diff.slang	New test file verifying backward-mode autodiff for fmod operation. Includes a differentiable test function, compute kernel with differential pair construction, and backward differentiation invocation via __bwd_diff. Expected derivatives documented as comments.
Test Expectations tests/autodiff/fmod-backward-diff.slang.expected.txt	New expected output file containing float type values and numerical results from the fmod backward-diff test execution.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 A fmod test hops into the fold,
Backward diff derivatives taking hold,
dx and dy dance in harmony,
Regression caught, now safe and free! 🌟

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding a failing test for a specific autodiff bug with issue reference.
Description check	✅ Passed	The description is directly related to the changeset, explaining the test's purpose, expected vs current behavior, and expected output.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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