`pixel` variable can be undefined 

**Describe the bug**

https://github.com/Project-MONAI/MONAI/blob/d0d720d0e9cea7d279cbf3cbb2066292f7efd333/monai/apps/pathology/transforms/post/array.py#L461

If none of the if/elif conditions are satisfied, `pixel` is undefined here.