Fix kvzip having poor performance

[maxjeblick]

PR description

Probable fix of #147.

Root cause of the issue:

• kwargs.get("past_key_values", None) returns DynamicCache(layers=[])
• bool(DynamicCache(layers=[])) equals to false, as bool method falls back to __len__ and len(DynamicCache(layers=[])) == len([]) = 0.
• Thus, self._cache=None, and it won't be used to perform kvzip compression.

Cause of this bug:
Transformers library 4.54. changed the naming convention from past_key_value to past_key_values.
During some transition period, however, there existed some back on forth on the naming convention ( transformers changed cache implementation a few times, see e.g .#104). I decided to make kvzip compatible with both naming conventions while working on #115 to allow for smoother devleopment. As it turned out, my implementation was faulty.

Comments:
There are other parts in kvzip that can be refactored. For this PR, I'll only address the actual bug fix to have better visibility.

Checklist

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[maxjeblick]

/ok to test 7520bdc

[maxjeblick]

Ruler 4k @ 50% cr, for qwen3-8b

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