[Fix] Change CustomDataset to BaseSegDataset (#1791)

demo/MMSegmentation_Tutorial.ipynb

mmseg/datasets/__init__.py

mmseg/datasets/ade.py

mmseg/datasets/chase_db1.py

mmseg/datasets/cityscapes.py

mmseg/datasets/coco_stuff.py

mmseg/datasets/custom.py

mmseg/datasets/drive.py

mmseg/datasets/hrf.py

mmseg/datasets/isaid.py

mmseg/datasets/isprs.py

mmseg/datasets/loveda.py

mmseg/datasets/pascal_context.py

mmseg/datasets/potsdam.py

mmseg/datasets/stare.py

mmseg/datasets/voc.py

tests/test_datasets/test_dataset.py

tests/test_datasets/test_dataset_builder.py