Fixed bug in multitarget macenko during import

[andreped]

Doing this was producing an error:

python -c "import torchstain; torchstain.normalizers.MultiMacenkoNormalizer(backend='torch')"

But after these changes, it is possible to use as with any other normalizer.

However, I am struggling to understand how to use it properly, as it just seem to crash all the time, but that can be handled in a separate issue/PR.

[andreped]

Any thoughts on this @desi-ivanov? Do you have a small code snippet showing how to use it? Currently, I just get an error when I use it on simple example images.

[carloalbertobarbano]

I'll merge the PR and take a look at the bug