warning added for single GPU and NCCL (#1226)

* issue link referred
* nccl required version mentioned

Author: Jaiaid

imagenet/main.py

@@ -106,8 +106,11 @@ def main():
 
     if torch.cuda.is_available():
         ngpus_per_node = torch.cuda.device_count()
+        if ngpus_per_node == 1 and args.dist_backend == "nccl":
+            warnings.warn("nccl backend >=2.5 requires GPU count>1, see https://github.com/NVIDIA/nccl/issues/103 perhaps use 'gloo'")
     else:
         ngpus_per_node = 1
+
     if args.multiprocessing_distributed:
         # Since we have ngpus_per_node processes per node, the total world_size
         # needs to be adjusted accordingly