[BUG] - python code error in ddp_tutorial

[rebornwwp]

Add Link

https://github.com/pytorch/tutorials/blob/main/intermediate_source/ddp_tutorial.rst#initialize-ddp-with-torchdistributedruntorchrun

Describe the bug

https://github.com/pytorch/tutorials/blob/main/intermediate_source/ddp_tutorial.rst#initialize-ddp-with-torchdistributedruntorchrun

os package is not imported insert import os in the first line

Describe your environment

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cc @wconstab @osalpekar @H-Huang @kwen2501