Fix timeout in MultimodalRunnerTest by limiting token generation

[rascani]

Summary: The testGemma and testVoxtral tests were timing out after 600 seconds because they could potentially generate hundreds of tokens without a hard limit. This adds maximumNewTokens = 128 to bound generation and temperature = 0 for deterministic output, preventing the tests from exceeding the timeout threshold.

Differential Revision: D90892374

[pytorch-bot]

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