Loss script fixes

[ericspod]

Fixes #1592.

Description

Fixes most of the loss functions to be compatible with Torchscript. Omits LocalNormalizedCrossCorrelationLoss, GlobalMutualInformationLoss, and BendingEnergyLoss as some significant code changes may be necessary. The changes made mostly are about dealing with Torchscript limitations regarding generator expressions or acceptable argument types. Union for example isn't compatible since statically-typed compiled code is targeted by Torchscript and this cannot handle this sort of polymorphism (I think).

One outstanding issue is that Enum classes are not compatible with Torchscript in Pytorch<1.7. The tests for Dice, Tversky, and others is only run for versions 1.7 and up.

Status

Ready

Types of changes

• 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 --codeformat --coverage.
• Quick tests passed locally by running ./runtests.sh --quick.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[ericspod]

/black

[wyli]

thanks, these are non-breaking enhancements for torchscript support