[Enhancement] Modify interface of MMSeginferencer and add docs

[MeowZheng]

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Motivation

Make MMSeginferencer easier to be used

Modification

1. Add _load_weights_to_model to MMSeginferencer, it is for get dataset_meta from ckpt
2. Modify and remove some parameters of __call__, visualization and postprocess
3. Add function of save seg mask, remove dump pkl.
4. Refine docstring of MMSeginferencer and SegLocalVisualizer
5. Add the user documentation of MMSeginferencer

BC-breaking (Optional)

yes, remove some parameters, we need to discuss whether keep them with deprecated waring or just remove them as the MMSeginferencer just merged in mmseg a few days.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Patch coverage: 72.58% and project coverage change: +0.03 🎉

Comparison is base (eca61d3) 83.44% compared to head (5d349ff) 83.48%.

❗ Current head 5d349ff differs from pull request most recent head 085c6bb. Consider uploading reports for the commit 085c6bb to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2658      +/-   ##
===========================================
+ Coverage    83.44%   83.48%   +0.03%     
===========================================
  Files          147      147              
  Lines         8620     8664      +44     
  Branches      1293     1301       +8     
===========================================
+ Hits          7193     7233      +40     
+ Misses        1205     1202       -3     
- Partials       222      229       +7     

Flag	Coverage Δ
unittests	83.48% <72.58%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
mmseg/apis/mmseg_inferencer.py	74.38% <71.66%> (+0.02%)	⬆️
mmseg/models/segmentors/cascade_encoder_decoder.py	91.66% <100.00%> (+0.17%)	⬆️
mmseg/visualization/local_visualizer.py	98.27% <100.00%> (ø)
mmseg/datasets/transforms/transforms.py	90.40% <0.00%> (+0.90%)	⬆️

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[csatsurnh]

For classes and palette docstring in local_visualizer, whether delete default to cityscapes in classes or add default to cityscapes in palette, not sure which is better.