question: How to use Accuracy with ignore class?
context: Please see same question on Stack Overflow.
  When using Accuracy with a class that should be ignored, meaning it has labels but can never be predicted, the scoring is wrong, because it is calculated with the never predicted labels that should be ignored.
  How to use Accuracy while ignoring some class?
  Thanks :)
answer: It is currently not supported in the accuracy metric, but we have an open PR for implementing that exact feature PyTorchLightning/metrics#155
  Currently what you can is instead calculate the confusion matrix and then ignore some classes based on that (remember that the true positive/correctly classified are found on the diagonal of the confusion matrix):
  ignore_index = 3
  metric = ConfusionMatrix(num_classes=3)
  confmat = metric(preds, target)
  confmat = confmat[:2,:2] # remove last column and row corresponding to class 3
  acc = confmat.trace() / confmat.sum()