[Features] Intialize dataset with ann_file

[MeowZheng]

Motivation

1. As title

Modification

1. Add ann_file and file_client_args in BaseDataset
2. Add load_ann_file in BaseDataset

[codecov]

Codecov Report

Merging #122 (0fe8eac) into master (579b68b) will increase coverage by 0.07%.
The diff coverage is 31.25%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
+ Coverage   68.78%   68.85%   +0.07%     
==========================================
  Files          92       92              
  Lines        5135     5150      +15     
  Branches      727      729       +2     
==========================================
+ Hits         3532     3546      +14     
  Misses       1489     1489              
- Partials      114      115       +1     

Flag	Coverage Δ
unittests	68.85% <31.25%> (+0.07%)	⬆️

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

Impacted Files	Coverage Δ
mmflow/datasets/base_dataset.py	80.00% <31.25%> (-13.34%)	⬇️
mmflow/datasets/pipelines/transforms.py	79.16% <0.00%> (+1.66%)	⬆️

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