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## Motivation

The focal Tversky loss was proposed in https://arxiv.org/abs/1810.07842.
It has nearly 600 citations and has been shown to be extremely useful
for highly imbalanced (medical) datasets. To add support for the focal
Tversky loss, only few lines of changes are needed for the Tversky loss.

## Modification

Add `gamma` as (optional) argument in the constructor of `TverskyLoss`.
This parameter is then passed to `tversky_loss` to compute the focal
Tversky loss.

## BC-breaking (Optional)

Does the modification introduce changes that break the
backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the
downstream projects should modify their code to keep compatibility with
this PR.

## Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases
here, and update the documentation.

## Checklist

1. Pre-commit or other linting tools are used to fix the potential lint
issues.
2. The modification is covered by complete unit tests. If not, please
add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects,
this PR should be tested with downstream projects, like MMDet or
MMDet3D.
4. The documentation has been modified accordingly, like docstring or
example tutorials.

Reopening of previous
[PR](https://github.com/open-mmlab/mmsegmentation/pull/2783).