[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