use of deprecated size_average=False in train_model

Training models gives the warning:

```
/usr/local/lib/python3.6/dist-packages/torch/nn/_reduction.py:44: UserWarning: size_average and reduce args will be deprecated, please use reduction='sum' instead.
  warnings.warn(warning.format(ret))
```

replacing line 28 of quick_train.py with 
```
   criterion = MSELoss(reduction='sum')
```
appears to be the recommended way to address this.