[Refactor] Refactor decode_head and segmentors and add preprocess_cfg

configs/_base_/datasets/ade20k.py

configs/_base_/datasets/ade20k_640x640.py

configs/_base_/datasets/chase_db1.py

configs/_base_/datasets/cityscapes.py

configs/_base_/datasets/cityscapes_1024x1024.py

configs/_base_/datasets/cityscapes_768x768.py

configs/_base_/datasets/cityscapes_769x769.py

configs/_base_/datasets/cityscapes_832x832.py

configs/_base_/datasets/coco-stuff10k.py

configs/_base_/datasets/coco-stuff164k.py

configs/_base_/datasets/drive.py

configs/_base_/datasets/hrf.py

configs/_base_/datasets/isaid.py

configs/_base_/datasets/loveda.py

configs/_base_/datasets/pascal_context.py

configs/_base_/datasets/pascal_context_59.py

configs/_base_/datasets/pascal_voc12.py

configs/_base_/datasets/potsdam.py

configs/_base_/datasets/stare.py

configs/_base_/datasets/vaihingen.py