test: add comprehensive gradient tests for CPU/GPU coverage

[GiggleLiu]

Summary

Add comprehensive gradient tests to ensure complete coverage across:

• CPU and GPU execution
• Non-batched and batched operations
• All three algebras (MaxPlus, MinPlus, MaxMul)

Test Coverage Matrix

Test Type	Non-Batched CPU	Non-Batched GPU	Batched CPU	Batched GPU
Gradient Structure	✅ All	✅ All	✅ All	✅ All
Numerical Gradient	✅ All	✅ All	✅ All	✅ MaxPlus
Optimization	✅ MaxPlus	✅ All	✅ MaxPlus	✅ MaxPlus
Accumulation	✅ MaxPlus	✅ MaxPlus	✅ MaxPlus	✅ MaxPlus
Sparsity	✅ MaxPlus	✅ MaxPlus	✅ MaxPlus	✅ MaxPlus

New Tests Added (19 total)

Batched CPU (3):

• test_batched_numerical_gradient_minplus
• test_batched_numerical_gradient_maxmul
• test_batched_gradient_sparsity

Non-Batched GPU (9):

• test_gpu_minplus_gradient
• test_gpu_maxmul_gradient
• test_gpu_gradient_accumulation
• test_gpu_gradient_sparsity
• test_gpu_numerical_gradient_maxplus
• test_gpu_numerical_gradient_minplus
• test_gpu_numerical_gradient_maxmul
• test_gpu_minplus_optimization
• test_gpu_maxmul_optimization

Batched GPU (7):

• test_batched_gpu_maxplus_gradient_structure
• test_batched_gpu_minplus_gradient_structure
• test_batched_gpu_maxmul_gradient_structure
• test_batched_gpu_numerical_gradient
• test_batched_gpu_optimization_convergence
• test_batched_gpu_gradient_accumulation
• test_batched_gpu_gradient_sparsity

Test Results

207 passed, 24 skipped

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.41%. Comparing base (a270d5d) to head (00b708b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #33   +/-   ##
=======================================
  Coverage   96.41%   96.41%           
=======================================
  Files          18       18           
  Lines         892      892           
=======================================
  Hits          860      860           
  Misses         32       32           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.