test: cover Compressor overlap and non-overlap windowing (PRPUNDIT-25)

[jiagaoxiang]

Same-repo refile of #1085 so Primus-CI-TAS runs against AMD-AGI/Primus (fork PRs do not receive Docker Hub credentials, so build-docker / torch unit tests never ran).

This PR was created by dougljia via Test Gap Resolver.

Summary

• Adds a dedicated CPU behavioral test for Compressor.forward in tests/unit_tests/backends/megatron/test_compressor_forward.py, not the FP32-policy suite (test_v4_keep_in_fp32.py) and not the fused-pool kernel suite (test_compressor_pool.py).
• Independent manual oracle for CSA overlap (ratio=4) and HCA non-overlap (ratio=128): window i is [half_a[i], half_b[i-1]] with zeros on window 0, or a contiguous ratio slice, then eager softmax-pool + RMSNorm.
• Also covers fused vs unfused projections (PRIMUS_COMPRESS_FUSE_PROJ) and the sequence-not-divisible-by-ratio guard, per linxwang's PRPUNDIT-25 note.

Closes test gap PRPUNDIT-25.

Test plan

• PYTHONPATH=. pytest tests/unit_tests/backends/megatron/test_compressor_forward.py (8 passed, CPU)
• black --check on the new file

[Copilot]

🟢 Approval recommended

The PR adds a well-scoped, CPU-only test suite that aligns with the implementation semantics and improves coverage without changing production code paths.

Pull request overview

Adds a dedicated, CPU-based behavioral oracle test suite for primus.backends.megatron.core.transformer.compressor.Compressor.forward, specifically validating correct window construction for CSA overlap (ratio=4) and HCA non-overlap (ratio=128), plus fused vs. unfused projection equivalence and the sequence-length divisibility guard.

Changes:

• Introduces a manual (helper-independent) reference implementation for windowing + softmax-pool + RMSNorm to validate Compressor.forward end-to-end on CPU.
• Covers both overlap and non-overlap behaviors, including a targeted check that overlap window 0 is causally padded (no predecessor influence).
• Adds a parity test ensuring PRIMUS_COMPRESS_FUSE_PROJ toggling does not change outputs (given equivalent weights), and asserts the expected “sequence not divisible by ratio” failure.

File summaries

File	Description
tests/unit_tests/backends/megatron/test_compressor_forward.py	New CPU behavioral tests providing an independent oracle for Compressor.forward windowing (overlap/non-overlap), fused/unfused projection parity, and input-length guard behavior.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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[jiagaoxiang]

TestIntel PR Steward: Checked CI on head 971757f (poll flagged build-docker (3.12) as failed).

That failure is stale: it's from workflow run #4994, which ran against the fork context (jiagaoxiang/Primus) before this PR was refiled into AMD-AGI/Primus directly — exactly the missing-Docker-Hub-credentials situation described in the PR body. The current, authoritative run for this same commit against AMD-AGI/Primus is #5026: code-lint and dependency-review have already passed, and build-docker (3.12) / run-unittest-jax are queued (not failed) there.

No code change needed on my end. I'll keep watching for the new run to finish and for review activity.