Support ignore_index in ConfusionMatrix

[idow09]

🚀 Feature

Add an ignore_index parameter to the classification metric ConfusionMatrix, as in Accuracy, Precision, Recall, etc...

Motivation

I use a MetricCollection for my classifier evaluation, and every other metric in this collection supports ignore_index except ConfusionMatrix. This causes a lot of trouble and I don't see a reason for not supporting it.

Pitch

ConfusionMatrix should support ignore_index so this should work totally fine:

target = torch.tensor([1, 1, 0, 0, -1])
preds = torch.tensor([0, 1, 0, 0, 1])
confmat = ConfusionMatrix(num_classes=2, ignore_index=-1)
confmat(preds, target)

and output:

tensor([[2, 0],
        [1, 1]])

Alternatives

I tried implementing my own IgnoreIndexConfusionMatrix but I encountered device synchronization issues...

[github-actions]

Hi! thanks for your contribution!, great first issue!

[SkafteNicki]

Issue will be fixed by classification refactor: see this issue #1001 and this PR #1195 for all changes

Small recap: Issue ask that ConfusionMatrix also supports ignore_index argument known from Accuracy, Precision, Recall . Is now supported in all the new introduced versions e.g. BinaryConfusionMatrix, MulticlassConfusionMatrix, MultilabelConfusionMatrix:

import torch
from torchmetrics.classification import BinaryConfusionMatrix

target = torch.tensor([1, 1, 0, 0, -1])
preds = torch.tensor([0, 1, 0, 0, 1])
confmat = BinaryConfusionMatrix(num_classes=2, ignore_index=-1)
confmat(preds, target)
# tensor([[2, 0],
#         [1, 1]])

Issue will be closed when #1195 is merged.