Add EntropyGatedChunkKVPress - #263

PR description

Adds EntropyGatedChunkKVPress (EG-ChunkKV), as proposed and approved in #261.

ChunkKVPress scores each chunk by its aggregate token importance and then keeps or drops that
chunk as a whole. Because the score is a magnitude statistic, it cannot distinguish a coherent
chunk, whose importance is spread across its tokens, from a spiky one, where a single needle
token holds nearly all the mass and the remaining chunk_length - 1 tokens are filler. Both can
score identically, and for the spiky chunk, keeping it whole spends chunk_length cache slots to
preserve one useful token — which, under a fixed budget, evicts a chunk that would otherwise be kept.

EG-ChunkKV adds a second, shape statistic computed from the same per-token scores ChunkKV already
has: the normalized within-chunk Shannon entropy H̃_i ∈ [0, 1], where 1 means coherent and 0 means
concentrated in a few tokens. Important-but-spiky chunks are reduced to their top-rescue_size
tokens rather than kept whole, and the freed budget is spent on further chunks. The number of
retained tokens is exactly max(1, floor((1 - compression_ratio) * kv_len)), identical to
ChunkKVPress, so the two are budget-matched by construction.

Selection procedure

1. S_i: the mean of the head-summed, non-negative token scores in chunk i. This is the same
   statistic ChunkKVPress ranks chunks by, so both methods order chunks identically.
2. H̃_i: the normalized Shannon entropy of those same token scores within the chunk.
3. The nuance: instead of taking a top-k over chunks up front, we fix the same token budget L
   that ChunkKV uses and walk chunks greedily in decreasing S_i, spending budget as we go:

important_i = S_i >= median(S)        spiky_i = H̃_i < τ        (τ defaults to median(H̃))

if important_i and spiky_i:   keep the chunk's top-`rescue_size` tokens    ← rescue the needle
elif chunk fits in budget:    keep the whole chunk                         ← coherent, keep whole
else:                         keep the top-`budget` tokens                 ← boundary truncation

If the rescues leave budget unspent, a final fill adds the highest-scoring not-yet-retained tokens,
so exactly L positions are kept and the retained indices are sorted back into original order.

Both gating masks are computed vectorized and moved to CPU once, so the sequential budget walk reads
no GPU scalars per iteration — without that, the per-chunk synchronization made this measurably
slower than plain ChunkKV.

Results

Llama-3.1-8B-Instruct, SnapKV as the inner scorer(for a fair comparison vs chunkkv, although we are even stronger with expected attention), chunk_length=10, rescue_size=4. cr is the
fraction of the KV cache removed. LongBench is the 16-task average; LOOGLE is ROUGE-L × 100. Higher
is better. ChunkKV is run at the same chunk_length=10 so the comparison isolates the entropy gate.

The gain holds at every ratio and grows as the budget tightens, peaking at +1.35 at
cr=0.90. LOOGLE shows the same direction under aggressive compression (+0.42 and +0.46) but is
slightly behind at cr=0.70 (−0.11), which is consistent with the mechanism: the wasted budget only
becomes costly once slots are genuinely scarce.

Notes for review

• No preprint yet. As mentioned in Entropy Gated ChunkKV (EG-ChunkKV) #261, we have work toward one but nothing released, so the
  docstring cites only the ChunkKV paper.
• default_presses checkbox is intentionally unticked. That list is instantiated as
  cls(compression_ratio=...), but this press requires press= and exposes compression_ratio as a
  delegating property, so it cannot be constructed that way. ChunkKVPress is absent from the list
  for the same reason. Instead the press is added to the wrapper_press matrix in
  tests/presses/test_presses.py, following the ChunkKVPress precedent, which exercises it against
  every press in default_presses.
• chunk_length defaults to 10, not 20 as in ChunkKVPress. Finer granularity gives the gate
  more chunks to reallocate budget between — the table above shows the mechanism is markedly stronger
  at 10 — and it is the configuration all reported numbers use (however we did measure our method at c=20 and it will be presented in our preprint in depth).

Checklist

Before submitting a PR, please make sure:

• Tests are working (make test)

• Code is formatted correctly (make style, on errors try fix with make format)

• Copyright header is included

• All commits are signed-off using git commit -s

• (new press) mypress_press.py is in the presses directory

• (new press) MyPress is in __init__.py

• (new press) README.md is updated with a 1 liner about the new press in the Available presses section

• (new press) New press is in the default_presses list in tests/default_presses.py

• (new press) A docstring is provided that follows the same structure as the existing ones