[V1] Delay all xgrammar usage until needed

[russellb]

PR #14575 delayed initialization of the grammar bitmask until it was
needed to try to fix a problem encountered on TPU systems.
Unfortunately, that change was not sufficient.

We need to delay usage of ALL xgrammar APIs, not just the grammar
initialization. This change implements that. More initialization is now
deferred until the first time a structured output request is received.

Signed-off-by: Russell Bryant [email protected]

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[NickLucche]

Working on my side (TPU). I also like the fact that you're only initializing when structured output is enabled.

[russellb]

Working on my side (TPU). I also like the fact that you're only initializing when structured output is enabled.

Thank you for the quick testing!

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @russellb.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[russellb]

rebased to resolve conflicts after #14622 went in