[Bugfix] Data parallel example might fail if the users script initializes torch.cuda

[Jackmin801]

This PR moves the setting of CUDA_VISIBLE_DEVICES to the main proc instead of having them set at the start of the target function. This prevents bugs caused by modifications to the script that initialize torch.cuda (e.g. importing flash attention)

[github-actions]

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[Jackmin801]

This PR didnt make any dependency changes so the docs build failure shouldnt be related

Downloading soupsieve-2.6-py3-none-any.whl (36 kB)
ERROR: THESE PACKAGES DO NOT MATCH THE HASHES FROM THE REQUIREMENTS FILE. If you have updated the package versions, please update the hashes. Otherwise, examine the package contents carefully; someone may have tampered with them.
    unknown package:
        Expected sha256 97787ff1fa3256a3eef9eda523a63dbf299f7b47e053cfcf684a1c2a8380c912
             Got        49b91c5c24ba4509dae434f256190740c6754b045cc5650ecf370a75199c147c