add StaticLayer.crop() to match DynamicLayer API

[ai-man-codes]

What does this PR do?

Fixes #43011

StaticLayer was missing a .crop(max_length) method, so implemented that according to the discussion of the issue.

Added StaticLayer.crop(max_length) to match the API of StaticCache with the DynamicCache

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case. → StaticLayer cache layer to implement .crop(seq_len) to match API of DynamicLayer #43011
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[ai-man-codes]

@zucchini-nlp I implemented StaticLayer.crop() method, can you review if this is implemented correctly? Thank You.

[qgallouedec]

Thanks @ai-man-codes the core is right; three things before we can merge:

• Negative max_length should mean "remove |max_length| tokens", as in DynamicLayer.crop — that's the API parity the issue asks for, and what assisted decoding uses. Right now it raises instead
• StaticSlidingWindowLayer inherits this and breaks: it keeps a second python counter (cumulative_length_int) that stays stale, and once the window is full the states are rolled, so [max_length:] isn't the tail. Needs an override that refuses to crop past the window, like DynamicSlidingWindowLayer.
• A test

Also needs a rebase: get_max_cache_shape is get_max_length now.