Macro Precision computation is wrong when ignore_index is used

[mpBarbato]

🐛 Bug

When ignore_index is used in MulticlassPrecision, it zeros out the metric on the class to ignore but it still includes the value in the average computation, leading to wrong results.

To Reproduce

import torch
from torchmetrics.classification import MulticlassPrecision

# simulate the output of a perfect predictor (i.e. preds == target)
target = torch.tensor([0, 1, 2, 0, 1, 2])
preds  = target.clone()

metric = MulticlassPrecision(num_classes=3, average='none', ignore_index=0)
res = metric(preds, target)
print(res)
# it prints [0., 1., 1.]

metric = MulticlassPrecision(num_classes=3, average='macro', ignore_index=0)
res = metric(preds, target)
print(res)
# it prints 0.6667 instead of 1

Expected behavior

It should not consider the ignored class in the average computation.

Environment

• TorchMetrics version: 0.11.4 installed from pip
• Python & PyTorch Version: 3.7.9 - 1.11.0+cu113
• Any other relevant information such as OS (e.g., Linux): Linux Mint 18.3 Sylvia

[github-actions]

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

[adamjstewart]

Duplicate of #1691