Fix RandomRotFlip interpolation for segmentation maps

[ziyangwang007]

Motivation

RandomRotFlip.random_rotate currently rotates segmentation fields with
mmcv.imrotate without specifying the interpolation mode. Since
mmcv.imrotate defaults to bilinear interpolation, rotating semantic
segmentation masks may introduce unexpected label values around class
boundaries.

Fixes #3399.

Modification

• Use interpolation='nearest' when rotating entries in seg_fields in
  RandomRotFlip.random_rotate.

BC-breaking (Optional)

No backward-incompatible change is expected. The change only makes
RandomRotFlip preserve discrete segmentation labels during rotation.

Use cases (Optional)

This affects training pipelines that use RandomRotFlip for semantic
segmentation masks. It prevents augmented masks from containing invalid class
ids produced by bilinear interpolation.

Checklist

1. The change is small and limited to RandomRotFlip.random_rotate.
2. Local pre-commit was not completed because the local pre-commit environment
   failed on the deprecated python_venv hook.
3. Local unit tests were not run because the local environment is missing
   mmcv.
4. No documentation change is required because this is a bug fix.