test: Update L0_openai TRT-LLM model repository preparation

[pskiran1]

What does the PR do?

Updates the qa/L0_openai TRT-LLM test setup to remove hard-coded version for downloading TRT-LLM backend models, which was introduced in the https://github.com/triton-inference-server/server/pull/8510/changes#diff-811fb0863c7b3d9caae9db6721260d3a26b3c8ea9c48a24cb2728aa86da928c9.

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• build
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• CI Pipeline ID: 48006607

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[Copilot]

Pull request overview

Updates the qa/L0_openai TRT-LLM test setup to rely on a pre-existing TRT-LLM model repository skeleton inside the container, rather than cloning TensorRT-LLM during the test run.

Changes:

• Removed the helper that cloned TensorRT-LLM to obtain triton_backend/all_models.
• Updated TRT-LLM preparation to copy the inflight batcher model repository from a container path (/app/all_models/...).
• Simplified prepare_tensorrtllm invocation by dropping the TensorRT-LLM repo directory argument.

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[yinggeh]

Not sure how /app/all_models is prepared. Isn't it better to clones from Triton forked repository https://github.com/triton-inference-server/tensorrtllm_backend? So that when the test breaks due to out-dated models, we can update our local repo and verify in the CI.

[yinggeh]

LGTM. Would like to get @mc-nv approval before merging