[Enhancement] Support inherit cityscapes dataset

[xiexinch]

Motivation

Support user custom dataset by inheriting CityscapesDataset class, the original implementation requires user's image suffix same with cityscapes, it's not flexible.

Modification

Set default value to the constructor function parameter.

Use cases (Optional)

@DATASETS.register_module()
class MyDataset(CityscapesDataset):
    """MyDatasett."""

    def __init__(self, **kwargs):
        super().__init__(
            img_suffix='_train.png',
            seg_map_suffix='_gt_mask.png',
            **kwargs)

[codecov]

Codecov Report

Merging #750 (4abb6d0) into master (e235c1a) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 4abb6d0 differs from pull request most recent head 231258c. Consider uploading reports for the commit 231258c to get more accurate results

@@           Coverage Diff           @@
##           master     #750   +/-   ##
=======================================
  Coverage   87.64%   87.64%           
=======================================
  Files         108      108           
  Lines        5886     5886           
  Branches      958      958           
=======================================
  Hits         5159     5159           
  Misses        535      535           
  Partials      192      192           

Flag	Coverage Δ
unittests	87.63% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmseg/datasets/cityscapes.py	85.00% <100.00%> (ø)

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