Fix complaints about 0-dim tensor

[gussmith23]

Traceback (most recent call last):
  File "mnist/train.py", line 88, in <module>
    loss.data[0], acc, optimizer.param_groups[0]['lr']))
IndexError: invalid index of a 0-dim tensor. Use tensor.item() to convert a 0-dim tensor to a Python number

and

Traceback (most recent call last):
  File "mnist/train.py", line 108, in <module>
    test_loss += F.cross_entropy(output, target).data[0]
IndexError: invalid index of a 0-dim tensor. Use tensor.item() to convert a 0-dim tensor to a Python number

[aaron-xichen]

good job~