Update import path for prepare_image function, fixes #15178

[nil-is-all]

Earlier, prepare_image() was in 'executorch.examples.models.llava.image_util', which does not exist anymore. Currently, the function exists in 'executorch.examples.models.llava.model'. Updated the import statement accordingly

Fixes #15178

cc @mergennachin @byjlw @iseeyuan @lucylq @helunwencser @tarun292 @kimishpatel @jackzhxng @larryliu0820 @cccclai

[pytorch-bot]

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❌ 2 New Failures, 1 Unrelated Failure

As of commit 57ca124 with merge base eb888f8 ():

NEW FAILURES - The following jobs have failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t b4e20de0139d7e9236fc6120f2495e6d4e389357c75b6f11a8a55230f1a23299 /exec failed with exit code 139
• Test CUDA Builds / test-model-cuda-e2e (mistralai, Voxtral-Mini-3B-2507, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t 046125cce40c24acafe191ce48173d67d9b26d3975f72758b559ad9dc65e8e52 /exec failed with exit code 1

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)

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