[Fix] Fix ignore class id from -1 to 255 in master (#2515)

## Motivation

This fixes #2493. When the `label_map` is created, the index for ignored
classes was being set to -1, whereas the index that is actually ignored
is 255. This worked indirectly since -1 was underflowed to 255 when
converting to uint8.

The same fix was made in the 1.x by #2332 but this fix was never made to
`master`.

## Modification

The only small modification is setting the index of ignored classes to
255 instead of -1.

## Checklist

- [x] Pre-commit or other linting tools are used to fix the potential
lint issues.
  - _I've fixed all linting/pre-commit errors._
- [x] The modification is covered by complete unit tests. If not, please
add more unit test to ensure the correctness.
- _No unit tests need to be added. Unit tests that are affected were
modified.
- [x] If the modification has potential influence on downstream
projects, this PR should be tested with downstream projects, like MMDet
or MMDet3D.
  - _I don't think this change affects MMDet or MMDet3D._
- [x] The documentation has been modified accordingly, like docstring or
example tutorials.
- _This change fixes an existing bug and doesn't require modifying any
documentation/docstring._

mmseg/datasets/custom.py

@@ -349,7 +349,7 @@ class CustomDataset(Dataset):
             self.label_map = {}
             for i, c in enumerate(self.CLASSES):
                 if c not in class_names:
-                    self.label_map[i] = -1
+                    self.label_map[i] = 255
                 else:
                     self.label_map[i] = class_names.index(c)
 
@@ -364,7 +364,7 @@ class CustomDataset(Dataset):
             palette = []
             for old_id, new_id in sorted(
                     self.label_map.items(), key=lambda x: x[1]):
-                if new_id != -1:
+                if new_id != 255:
                     palette.append(self.PALETTE[old_id])
             palette = type(self.PALETTE)(palette)
 

tests/test_data/test_loading.py

@@ -187,7 +187,7 @@ class TestLoading(object):
             # classes=["A", "C", "D"] which removes class "B".
             label_map={
                 0: 0,
-                1: -1,  # simulate removing class 1
+                1: 255,  # simulate removing class 1
                 2: 1,
                 3: 2
             },
@@ -204,7 +204,7 @@ class TestLoading(object):
 
         true_mask = np.ones_like(gt_array) * 255  # all zeros get mapped to 255
         true_mask[2:4, 2:4] = 0  # 1s are reduced to class 0 mapped to class 0
-        true_mask[2:4, 6:8] = -1  # 2s are reduced to class 1 which is removed
+        true_mask[2:4, 6:8] = 255  # 2s are reduced to class 1 which is removed
         true_mask[6:8, 2:4] = 1  # 3s are reduced to class 2 mapped to class 1
         true_mask[6:8, 6:8] = 2  # 4s are reduced to class 3 mapped to class 2