[v0][Core] Use xgrammar shared context to avoid copy overhead for offline engine

[sethkimmel3]

The deepcopy introduced in #11637 adds a lot of overhead when adding a large number of requests to an llm_engine. This adds a more efficient method of copying the XGrammarLogitsProcessor data structure to remove that overhead.

cc: @mgoin @aarnphm

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[aarnphm]

should it be

Suggested change

	if hasattr(self, 'token_bitmask') and self.token_bitmask is not None:
	new_processor.token_bitmask = xgr.allocate_token_bitmask(
	self.batch_size, self.config.vocab_size)
	if hasattr(self, 'token_bitmask') and self.token_bitmask is not None:
	new_processor.token_bitmask = self.token_bitmask

[aarnphm]

one tiny comment, if it passes the tests then LGTM.

[aarnphm]

@sethkimmel3 there are a few pre-commit problem can you fix this? thanks.

[aarnphm]

I cant update the title, but can you make it to [v0][Core] Use shared context to avoid copy overhead for offline engine

otherwise I think this should be ready to bring out of draft

[sethkimmel3]

Done and done @aarnphm!

[aarnphm]

Thanks. Once all PR pass we can merge this