UserWarning: The number of elements in the out tensor of shape [1] is 1

I am using HuggingFace pytorch-transformers and one of my pre-trained models refuse to fine tune giving me those UserWarnings for every torch.utils.data.DataLoader call.

I have described the details in https://stackoverflow.com/questions/60218634/userwarning-the-number-of-elements-in-the-out-tensor-of-shape-1-is-1

But someone downgraded my question for some reason. What am I missing?

Here is my Notebook so you can run and see the results:

Thanks for your help!