Accuracy Formula Not Correct?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D-ML/issues).
- [X] I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Looking at the [code](https://github.com/isl-org/Open3D-ML/blob/master/ml3d/tf/modules/metrics/semseg_metric.py) for the SemSegMetric class, I'm confused as to why this formula, acc = tp / (tp + fn), is used for accuracy when the formula for accuracy is acc = (tp + tn) / (tp + fp + tn + fn).

tp / (tp + fn) is instead the formula for recall.

Thanks.