[Fix] Remove dependcy mmdet when do not use `MaskFormerHead` and `MMDET_Mask2FormerHead` (#2448)

## Motivation

Calling `mmseg.utils.register_all_modules` will import `MaskFormerHead`
and `Mask2FormerHead`, it will crash if mmdet is not installed as `None`
cannot be initialized.

## Modification

- Modify `MMDET_MaskFormerHead=BaseModule` and `MMDET_Mask2FormerHead =
BaseModule` when cannot import from mmdet

mmseg/models/decode_heads/mask2former_head.py

@@ -4,12 +4,13 @@ from typing import List, Tuple
 import torch
 import torch.nn as nn
 import torch.nn.functional as F
+from mmengine.model import BaseModule
 
 try:
     from mmdet.models.dense_heads import \
         Mask2FormerHead as MMDET_Mask2FormerHead
 except ModuleNotFoundError:
-    MMDET_Mask2FormerHead = None
+    MMDET_Mask2FormerHead = BaseModule
 
 from mmengine.structures import InstanceData
 from torch import Tensor

mmseg/models/decode_heads/maskformer_head.py

@@ -4,11 +4,12 @@ from typing import List, Tuple
 import torch
 import torch.nn as nn
 import torch.nn.functional as F
+from mmengine.model import BaseModule
 
 try:
     from mmdet.models.dense_heads import MaskFormerHead as MMDET_MaskFormerHead
 except ModuleNotFoundError:
-    MMDET_MaskFormerHead = None
+    MMDET_MaskFormerHead = BaseModule
 
 from mmengine.structures import InstanceData
 from torch import Tensor