[Feature] Support COCO-LVIS dataset

[xiexinch]

Motivation

Support COCO-LVIS dataset

Paper: Scaling Semantic Segmentation Beyond 1K Classes on a Single GPU Repo: ESSNet

Modification

1. add convert tool to generate gt_mask
2. add COCOLVISDataset class
3. modify LoadAnnotations pipeline to support loading 16-bit label

BC-breaking (Optional)

Just add a optional parameter to LoadAnnotations method.

Checklist

TODO

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[bot]]

Codecov Report

Merging #977 (c94069d) into master (54bd4bd) will decrease coverage by 0.24%. The diff coverage is 47.36%.

@@            Coverage Diff             @@
##           master     #977      +/-   ##
==========================================
- Coverage   89.85%   89.61%   -0.25%     
==========================================
  Files         118      120       +2     
  Lines        6558     6596      +38     
  Branches     1019     1023       +4     
==========================================
+ Hits         5893     5911      +18     
- Misses        464      484      +20     
  Partials      201      201              

Flag	Coverage Δ
unittests	89.61% <47.36%> (-0.25%)	:arrow_down:

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

Impacted Files	Coverage Δ
mmseg/datasets/coco_lvis.py	41.17% <41.17%> (ø)
mmseg/datasets/__init__.py	100.00% <100.00%> (ø)
mmseg/datasets/misc.py	100.00% <100.00%> (ø)
mmseg/datasets/pipelines/loading.py	98.55% <100.00%> (+0.02%)	:arrow_up:

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[Junjun2016]

Hi @xiexinch The CI has been fixed, please merge the master branch.

[RockeyCoss]

Please merge the master branch into your branch, thank you.

[CLAassistant]

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