FIX Correctly call element_size (#1635)
Should fix the error introduced by #1630.
AFAICT, element_size should be called on the parameter, not the dtype.
Unfortunately, I had issues getting older PyTorch versions to work with
bnb, so I haven't tested the initial issue.
To be safe, I also re-added the previous code path using itemsize,
although it might be unnecessary (we would have to check the PyTorch
code to verify when the different attributes/methods were added).
