[Enhancement] Refactor VOT, UAV123, OTB100 and LaSOT dataset class based on BaseSOTDataset

[JingweiZhang12]

Refactor VOT2018Dataset, UAV123Dataset, OTB100Dataset and LaSOTDataset based on BaseSOTDataset.
Please refer to #393 for more details.

[GT9505]

Please fix the lint error firstly

[JingweiZhang12]

My local pre-commit doesn't report this lint error, so werid!

[GT9505]

This PR will be reviewed after PR #402 is merged

[GT9505]

This PR will be reviewed after PR #402 is merged

We can continue on this PR now.

[GT9505]

Please fix the CI before next review.

[codecov]

Codecov Report

Merging #401 (27f56a5) into master (142922e) will increase coverage by 0.53%.
The diff coverage is 86.12%.

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
+ Coverage   70.37%   70.91%   +0.53%     
==========================================
  Files         116      116              
  Lines        6113     6226     +113     
  Branches     1181     1192      +11     
==========================================
+ Hits         4302     4415     +113     
  Misses       1453     1453              
  Partials      358      358              

Flag	Coverage Δ
unittests	70.81% <86.12%> (+0.49%)	⬆️

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

Impacted Files	Coverage Δ
mmtrack/apis/train.py	16.66% <0.00%> (ø)
mmtrack/datasets/otb_dataset.py	65.11% <64.28%> (-34.89%)	⬇️
mmtrack/core/evaluation/eval_sot_ope.py	97.87% <85.71%> (+6.38%)	⬆️
mmtrack/datasets/vot_dataset.py	85.22% <88.23%> (-3.67%)	⬇️
mmtrack/datasets/base_sot_dataset.py	86.25% <90.90%> (+24.65%)	⬆️
mmtrack/core/evaluation/eval_sot_vot.py	86.23% <100.00%> (-0.25%)	⬇️
mmtrack/datasets/lasot_dataset.py	100.00% <100.00%> (ø)
mmtrack/datasets/uav123_dataset.py	100.00% <100.00%> (ø)
mmtrack/datasets/sot_test_dataset.py	23.91% <0.00%> (-54.35%)	⬇️
mmtrack/models/reid/base_reid.py	78.26% <0.00%> (-8.70%)	⬇️
... and 4 more

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[GT9505]

Please test the performance of SiameseRPN++ on VOT, UAV123, OTB100 and LaSOT after completing the reviewing comments. Then, we can merge this PR.