The error points to a batchnorm layer, which wants to calculate the running statistics, but fails due to a single provided sample.
This is usually the case, if you feed a batch to the model containing one sample.
Since you are using nn.DataParallel this could happen, if the batch size equals the number of GPUs.
Also, the last batch might be smaller, which could trigger this effect. In that case, you can skip the last (smaller) batch using drop_last=True in your DataLoader.