wrong cross entropy definition

cross_entropy function: https://github.com/minerva-ml/steppy-toolkit/blob/master/steppy_toolkit/pytorch/validation.py#L24 probably should be defined in a slightly different way:

```
def cross_entropy(output, target, squeeze=False):
    if squeeze:
        target = target.squeeze(1)
    return F.nll_loss(F.log_softmax(output), target)
```

Please verify.