Issue description

The loss plugin generates this warning with PyTorch 0.4 :

torch/utils/trainer/plugins/loss.py:8: UserWarning: invalid index of a 0-dim tensor. This will be an error in PyTorch 0.5. Use tensor.item() to convert a 0-dim

Here's a link to the offending line:
https://github.com/pytorch/pytorch/blob/master/torch/utils/trainer/plugins/loss.py#L8

It appears that this code was not migrated to PyTorch 0.4 as recommended here:
https://pytorch.org/2018/04/22/0_4_0-migration-guide.html

Proposed Fix

That line should be changed from:
return loss[0]
to
return loss.item()

System Info

This is using PyTorch 0.4.0.