[bugfix] add seed in torchrun_example.py

[youkaichao]

fixes #15895

#14274 updated the tests/distributed/test_torchrun_example.py , but didn't update torchrun_example.py .

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[WoosukKwon]

LGTM. Can we also raise an error when the external_launcher is used without a seed?

[youkaichao]

Can we also raise an error when the external_launcher is used without a seed?

done in 9369ba5