[executorch][llm] Make Gemma Apple test deterministic

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [executorch][devtools] Reset profiler state in ETDump tests #21410
• -> [executorch][llm] Make Gemma Apple test deterministic #21403

MultimodalRunnerTest.testGemma asserted on generated text while inheriting the default sampling temperature of 0.8. Set the temperature to zero for both generation calls so all four TestInfra configurations use deterministic greedy decoding. This commit was authored with Claude.

Differential Revision: D113786993

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⏳ 1 Pending, 1 Unrelated Failure

As of commit b45b908 with merge base 16b780b ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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