warning added for single GPU and NCCL

[Jaiaid]

• issue link referred to hint at single GPU user what to do if nccl internal check error
• nccl required version mentioned

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[Jaiaid]

re-committing this change with slight modification of warning rather than assertion. Also the GPU count variable is set to one to work with later world_size variable initiation. Although I think semantically it is confusing for CPU only platform.
This is done to help users avoid some possible frustration when using default dist backend which is NCCL.

[sebastian-burlacu]

Does this warning imply that the default backend is nccl? The '--dist-backend' argument could be documented better, in that case - as I would assume the first example given, with no --dist-backend specified, should work fine without giving a warning. As this is a warning, I am also wary of the 'requires' since that would imply it errors out if used incorrectly, but it still seems to work fine?

[Jaiaid]

@sebastian-burlacu
Yes the default backend is nccl, see line 68
This warning is just to give users a context where things may go wrong.
I may have mentioned the version requirement of nccl wrong but this error happened to me and relevant issue link is provided with the warning.