Add Unified Focal Loss

[JohnMasoner]

Fixes #4413

Description

Add Unified Focal Loss, about the issue

Status

Work in progress

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 -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.

[wyli]

thanks, please sign the commit https://github.com/Project-MONAI/MONAI/blob/dev/CONTRIBUTING.md#signing-your-work there are also some minor typing issues https://github.com/Project-MONAI/MONAI/runs/6858724085?check_suite_focus=true#step:7:532

[wyli]

Hi @danieltudosiu, could you help review this one if you have some time? thanks!

[danieltudosiu]

@wyli will have a look over the weekend.

[JohnMasoner]

@wyli Thank you for reviewing the code. I'm thinking about whether to add multiple categories or just support binary segmentation only.

[wyli]

thanks, I think the initial support of the binary cases is good (as long as we have the docstring and input check), we can create a new feature request/PR for multiclass extension

[wyli]

thanks, I only have a few minor comments about the documentation, otherwise we can merge these new modules.

[JohnMasoner]

@wyli thanks, you took the time to review my code. this has been a great help to me.

[wyli]

/build

[wyli]

/build

[wyli]

thanks, the tests look good, please help fix the typos then I'll merge this one.

[wyli]

thanks!

[wyli]

/build