ray init does not automaticly detect GPUs

[alessiovignoli]

ray is first initialized explicitly to set maximum resources limits, becuase it does not respect nextflow and cluster limits otherwise.

But doing so if num_gpus is left to None, default for ray.init see here -> docs the GPUs are not automaticly detected and used. In theory it should detect them all and use them all, but is not doing so.

[mathysgrapotte]

behaviour : if user hasn't specified gpus, should throw a useful warning