fix(tf): preserve ghost gradients in ProdForceGrad

[njzjz-bot]

Summary

• align legacy ProdForceGrad with ProdForce's [nframes, 3 * nall] output contract
• preserve each ghost atom's own upstream gradient instead of folding indices modulo nloc
• validate neighbor indices and cover a second ghost at nloc + 1 across two frames

Why existing tests missed this

The direct TensorFlow gradient test exercised the newer ProdForceSeA helper with nall == nloc, so it had neither ghost atoms nor the legacy ProdForceGrad implementation. Higher-level loc-frame coverage did not differentiate the legacy force output with deliberately distinct local and ghost upstream gradients. The stale code also looked superficially consistent with the newer local-only helper even though the legacy forward op had been changed in 2018 to retain separate ghost-force outputs.

The regression invokes tf.gradients on the real legacy ProdForce op, uses j_idx == nloc + 1 so the removed j_idx > nloc modulo branch is actually exercised, assigns distinct local and two-ghost upstream gradients, and uses two frames to lock the corrected nall * 3 frame stride.

Validation

• DP_ENABLE_PYTORCH=0 uv pip install -e .[cpu,test]
• pytest source/tests/tf/test_prod_force_grad.py -q using the current editable finder (3 passed, 1 inherited TensorFlow helper skipped)
• review follow-up against the rebuilt TensorFlow op: strengthened second-ghost regression passed (1 passed, 1 inherited helper skipped)
• ruff format .
• ruff check .
• clang-format dry run with --Werror
• git diff --check

Fixes #5656

Coding agent: Codex
Codex version: codex-cli 0.144.4
Model: gpt-5.6-sol
Reasoning effort: xhigh

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Review details

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 093acb2a-7259-4731-bd52-7dd96e6c3320

📥 Commits

Reviewing files that changed from the base of the PR and between 6c3b985 and 86b61b1.

📒 Files selected for processing (2)

• source/op/tf/prod_force_grad.cc
• source/tests/tf/test_prod_force_grad.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.32%. Comparing base (6c3b985) to head (86b61b1).

Files with missing lines	Patch %	Lines
source/op/tf/prod_force_grad.cc	33.33%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5848      +/-   ##
==========================================
- Coverage   78.58%   78.32%   -0.27%     
==========================================
  Files        1050     1050              
  Lines      120637   120642       +5     
  Branches     4356     4365       +9     
==========================================
- Hits        94801    94488     -313     
- Misses      24278    24592     +314     
- Partials     1558     1562       +4     

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