cpu_model_runner.py: skip the warm up if CompilationMode.NONE

[yamt]

for cpu backend, there seems to be little reason to perform warm up unless compiling.

Purpose

speed up tests.

Test Plan

Test Result

this reduced the wall time of cached runs of
examples/disaggregated/example_connector/run.sh
on my cpu-only environment from 85 seconds to 68 seconds.

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[claude]

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[bigPYJ1151]

Thanks :) LGTM