Avoid converting label ids twice by label map during evaluation (#1417)

* Avoid converting label ids twice by label map during evaluation

* Update mmseg/datasets/custom.py

Co-authored-by: Miao Zheng <76149310+MeowZheng@users.noreply.github.com>

* lint

Co-authored-by: Miao Zheng <76149310+MeowZheng@users.noreply.github.com>
Co-authored-by: MeowZheng <meowzheng@outlook.com>