fixes DiceCELoss for multichannel targets

[myron]

Fixes DiceCELoss for multichannel targets. Currently if "target" (ground truth label) is provided as a multichannel data (each channel is binary or float), then current DiceCELoss attempts to convert it to 1-channel using argmax (which could be impossible with overlapping labels). There is no need for argmax, since pytorch's cross entropy can handle multi-channel targets already.

A few sentences describing the changes proposed in this pull request.

Types of changes

• [x] Non-breaking change (fix or new feature that would not break existing functionality).
• [ ] Breaking change (fix or new feature that would cause existing functionality to change).
• [ ] New tests added to cover the changes.
• [ ] Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• [ ] Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• [ ] In-line docstrings updated.
• [ ] Documentation updated, tested make html command in the docs/ folder.

[myron]

added check for pytorch version

[wyli]

/black

[wyli]

/build

[myron]

some checks are failing, but it's something unrelated

[wyli]

/build