[Feature] Support dataset initialization with file_client

[MeowZheng]

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Motivation

Support to initialize dataset from file_client

Modification

1. Add file_client_args in __init__ with default value dict(backend='disk')
2. scan directory with file_client.list_dir_or_file in load_annotations
3. Read split file with mmcv.list_from_file in load_annotations

Use cases (Optional)

file_client_args = dict(
    backend='petrel',
    path_mapping=dict({
        '.data/cityscapes/': 'openmmlab:s3://openmmlab/datasets/segmentation/cityscapes/',
        'data/cityscapes/': 'openmmlab:s3://openmmlab/datasets/segmentation/cityscapes/'
    }))

data = dict(
...
    train=dict(
    ...
           file_client_args=file_client_args),
    val=dict(
    ...
        gt_seg_map_loader_cfg=dict(file_client_args=file_client_args),
        file_client_args=file_client_args),
    test=dict(
    ...
        gt_seg_map_loader_cfg=dict(file_client_args=file_client_args),
        file_client_args=file_client_args))

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

Merging #1402 (919d3e7) into master (3d0c2eb) will decrease coverage by 0.00%.
The diff coverage is 70.58%.

@@            Coverage Diff             @@
##           master    #1402      +/-   ##
==========================================
- Coverage   90.39%   90.39%   -0.01%     
==========================================
  Files         133      133              
  Lines        7906     7903       -3     
  Branches     1318     1318              
==========================================
- Hits         7147     7144       -3     
  Misses        536      536              
  Partials      223      223              

Flag	Coverage Δ
unittests	90.39% <70.58%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
mmseg/datasets/chase_db1.py	77.77% <0.00%> (-2.23%)	⬇️
mmseg/datasets/drive.py	77.77% <0.00%> (-2.23%)	⬇️
mmseg/datasets/hrf.py	77.77% <0.00%> (-2.23%)	⬇️
mmseg/datasets/pascal_context.py	75.00% <0.00%> (-1.48%)	⬇️
mmseg/datasets/custom.py	94.73% <100.00%> (+0.05%)	⬆️
mmseg/datasets/isaid.py	93.93% <100.00%> (-0.18%)	⬇️
mmseg/core/evaluation/class_names.py	88.33% <0.00%> (ø)

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[RockeyCoss]

LGTM

[Junjun2016]

LGTM.
This month, we can do regression benchmarks with ceph to verify this PR.