Added ignore_index param to forward(),
also implemented one hot encoding to ensure the dims of target matches
pred.
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## Motivation
Please describe the motivation of this PR and the goal you want to
achieve through this PR.
Attempted to solve the problems mentioned by #3172
## Modification
Please briefly describe what modification is made in this PR.
Added ignore_index into forward function (although the dice loss itself
does not actually take account for it for some reason).
Added _expand_onehot_labels_dice, which takes the target with shape [N,
H, W] into [N, num_classes, H, W].
## 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.
This is my first time contributing to open-source code, so I might have
made some stupid mistakes. Please don't hesitate to point it out.
* support reading class_weight from file in loss function
* add unit test of loss with class_weight from file
* minor fix
* move get_class_weight to utils
