make dynunet and mednext api compatible in deep supervsion mode

[elitap]

Fixes #8315

Description

If deep supervision is set to true the DynUNet model returns a tensor with the second dimension containing the upscaled intermediate outputs of the model, whereas the new MedNeXt implementation returns a tuple with the tensors of the intermediate outputs in their original low res.

Types of changes

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