Enhance docstring for ToDeviced

[yiheng-wang-nv]

It seems ToDeviced cannot work when num_workers > 1:

RuntimeError: Cannot re-initialize CUDA in forked subprocess. To use CUDA with multiprocessing, you must use the 'spawn' start method

Shall we add the docstring for this transform, thus users can avoid meeting error?

[Nic-Ma]

Thanks for raising it.
I will try to enhance the doc-string ASAP.

Thanks.