[fix] check before calling _specify_ddp_gpu_num

[min-xu-ai]

• this function is being removed in pytorch
• we only need to call it in case we are working with older pytorch
• internal diff: https://www.internalfb.com/diff/D27866440?entry_point=2

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What does this PR do?

Fixes # (issue).

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[blefaudeux]

Thanks Min, looks great

[wayi1]

Thanks for the fix!