fix: clamp n_kept to >=1 to prevent silent cache emptying on short contexts 🤖🤖🤖

[SuperMarioYL]

PR description

ScorerPress.compress (and the wrapper presses that re-derive their own n_kept) compute the
number of KV pairs to keep as int(k_len * (1 - compression_ratio)) with no floor guard. On a
short context (k_len of 1 or 2) with any non-zero compression_ratio, this floors to
n_kept = 0, so scores.topk(0) returns empty indices, keys.gather(2, …) writes an empty
(bsz, heads, 0, head_dim) cache, and the following decode step attends zero keys — producing
NaN / divergent output with no error raised.

This is the same "silent eviction" family as #227 (which fixed the pad-value +1), but on the
n_kept floor axis: chunk_press.py:77 and finch_press.py:107 already guard with
max(1, int(...)), so the maintainers clearly intend the floor — it was simply never backported
to the base ScorerPress.compress nor to the wrappers that re-derive n_kept. Notably
finch_press.py guards the chunk path (:107) but not the non-chunk path (:100), even
though both were added in the same PR #139 — an oversight, not a design choice.

This PR applies the existing max(1, int(...)) idiom in-place at every unguarded site:

file:line	scope
kvpress/presses/scorer_press.py:94	base ScorerPress.compress (propagates to all ScorerPress subclasses)
kvpress/presses/block_press.py:66	BlockPress re-derives n_kept
kvpress/presses/merging_press.py:86	MergingPress (docstring: "Identical to ScorerPress.compress except…")
kvpress/presses/key_rerotation_press.py:146	KeyRerotationPress (q_len form)
kvpress/presses/finch_press.py:100	FinchPress non-chunk path (the chunk path :107 is already guarded)
kvpress/presses/adakv_press.py:64	AdaKVPress (# ScorerPress definition copy)
kvpress/presses/criticalkv_press.py:149	CriticalAdaKVPress (# ScorerPress definition copy)

The change is purely additive: max(1, X) is identical to X whenever X >= 1, so for any
context long enough that int(k_len * (1 - ratio)) >= 1 the behaviour is unchanged. It only ever
lifts n_kept from 0 to 1 on degenerate short inputs, preventing the silent empty cache.

Two related sites are deliberately excluded from this PR to keep it a clean, reviewable
consistency fix of the ScorerPress idiom:

• decoding_press.py carries the same pattern but the decoding subsystem has active in-flight
  branches; it is left untouched here to avoid collisions and can be handled in a follow-up.
• kvcompose_press.py uses a composite_scores.numel() form rather than k_len; noted for a
  follow-up rather than mixed into this idiom-consistency sweep.

Refs #227.

Checklist

Before submitting a PR, please make sure:

• Tests are working — added tests/presses/test_scorer_press.py (parametrized, CPU-only,
  no model download); make test runs in CI.
• Code is formatted correctly (make style: flake8 clean on all touched files).
• Copyright header is included (SPDX header on the new test file).
• All commits are signed-off using git commit -s (DCO).

(n/a — not a new press: no __init__/README/default_presses/docstring changes.)

🤖🤖🤖

Summary by CodeRabbit

• Bug Fixes

  • Compression now retains at least one key/value pair, even with extremely high compression settings or very short sequences.
  • Prevented empty cache selections that could cause compression failures.

• Tests

  • Added regression coverage across supported compression methods to verify non-empty, valid key caches.

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@CodeRabbit full review

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✅ Action performed

Full review finished.

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Walkthrough

Compression presses now retain at least one KV pair when compression would otherwise select zero tokens. Parameterized regression tests cover all supported presses with one- and two-token contexts.

Changes

KV retention safeguards

Layer / File(s)	Summary
Minimum retention clamps kvpress/presses/*_press.py	Seven press implementations clamp retained-token counts to at least one before KV selection.
Short-context regression tests tests/presses/test_scorer_press.py	Parameterized tests exercise all supported presses and verify non-empty, finite caches for short contexts.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🔵 Low · up to 0603d

The PR clamps retained KV entries to at least one on short contexts, preventing empty-cache attention and related NaN or divergent output. It is mergeable with explicit owner awareness, but the regression test should also verify that both returned KV tensors are non-empty and finite.

Poem

A rabbit checks the tokens tight,
One KV pair remains in sight.
Short contexts no longer flee,
Tests hop through each press happily.
The cache stays finite, bright, and free.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the main fix: clamping n_kept to prevent empty KV caches for short contexts.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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Actionable comments posted: 1

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Inline comments:
In `@tests/presses/test_scorer_press.py`:
- Around line 107-111: Update the press.compress call to capture out_values
alongside out_keys, then assert out_keys and out_values each retain at least one
token and contain only finite values using the existing shape and NaN checks as
the model.

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Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between 4e41f14 and 0603dec.

📒 Files selected for processing (8)

• kvpress/presses/adakv_press.py
• kvpress/presses/block_press.py
• kvpress/presses/criticalkv_press.py
• kvpress/presses/finch_press.py
• kvpress/presses/key_rerotation_press.py
• kvpress/presses/merging_press.py
• kvpress/presses/scorer_press.py
• tests/presses/test_scorer_press.py

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