[Bugfix] Fix CFGGuide and use outlines for grammars that can't convert to GBNF

[mgoin]

@afeldman-nm found this issue while trying the offline and online structured output examples like python examples/offline_inference_structured_outputs.py. We were failing to convert the sql lark grammar into gbnf format for xgrammar and had that check+conversion in the wrong place to fallback to another backend.

It turns out that our outlines backend actually regressed on this example as well with the upgrade to 0.1.11 (it works fine if I use outlines==0.0.46). I made this PR to make xgrammar fallback to outlines in this case, and get outlines back to functional for it.

[github-actions]

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