Copy the template resolution logic from the base apply_chat_template to Voxtral

[Rocketknight1]

Fixes #45084. We need to resolve the chat template in the Voxtral code to avoid None being passed to _get_template_variables()!

cc @zucchini-nlp, follow-up to #44881

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[zucchini-nlp]

Great thanks a lot, caught up with model release and forgot about a failing test

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: voxtral

[Rocketknight1]

@zucchini-nlp my bad, I actually misread the Voxtral processor. Almost all of the actual handling is done inside self.tokenizer.apply_chat_template(). I think the right solution is just to leave the code alone, but fix _get_template_variables() so it doesn't crash when an empty template is passed. We can't really figure out the template variables for Voxtral anyway, since I think it uses mistral_common rather than jinja.

Updated the code to just handle that instead! Can you take another look and I'll merge if you're happy?

[zucchini-nlp]

Hmm I guess voxtral has no special kwargs expected for jinja, right? Should be fine in that case, otherwise we might need to manually define template_kwargs

Those jinja-kwargs can't be passed over to processor, it'll return ValueError at some point

[Rocketknight1]

run-slow: voxtral

[github-actions]

Workflow Run ⚙️

This comment contains run-slow, running the specified jobs:

models: ["models/voxtral"]
quantizations: []

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CI Results

Workflow Run ⚙️

Commit Info

Context	Commit	Description
RUN	fe64f6e5	workflow commit (merge commit)
PR	b5bd46a8	branch commit (from PR)
main	c9f1b95a	base commit (on main)

Model CI Report

❌ 4 new failed tests from this PR 😭

• voxtral:
  tests/models/voxtral/test_modeling_voxtral.py::VoxtralForConditionalGenerationIntegrationTest::test_mini_multi_turn_text_and_audio (❌ ⟹ ❌)
  tests/models/voxtral/test_modeling_voxtral.py::VoxtralForConditionalGenerationIntegrationTest::test_mini_single_turn_audio_only (❌ ⟹ ❌)
  tests/models/voxtral/test_modeling_voxtral.py::VoxtralForConditionalGenerationIntegrationTest::test_mini_single_turn_text_and_audio (❌ ⟹ ❌)
  tests/models/voxtral/test_modeling_voxtral.py::VoxtralForConditionalGenerationIntegrationTest::test_mini_single_turn_text_and_multiple_audios_batched (❌ ⟹ ❌)

[Rocketknight1]

Those tests are all failing already on main, so I don't think they should block this! We might want to check them in a future PR