[Bugfix]: import pandas for benchmarks

[ZhangShuaiyi]

FIX #19078

VLLM_USE_PRECOMPILED=1 uv pip install --editable ., pandas (introduced in #18511) is not installed.

[ZhangShuaiyi]

cc @ekagra-ranjan @ywang96

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[Isotr0py]

Thanks for your contribution! Since there are also other packages like datasets introduced for benchmark only, I think it would be better to make them as extra dependency (#19089). So that user can install them only when having demands for benchmark.