llm_runner: add Engine and Session interfaces (#20243) #14034

Job	Run time
CI run decision / decide	30s
Get changed files / get-changed-files	5s
unittest-cuda	0s
test-cuda-pybind	0s
check-all-cuda-builds	0s
export-model-cuda-artifact	0s
test-executorch-cuda-build-${{ matrix.cuda-version }}	0s
test-models-cuda	0s
test-model-cuda-e2e	0s
	35s

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