[Fix] fix several bugs in reid

[ToumaKazusa3]

This PR fixes several bugs in reid:

1. reid model can not be tested.
2. the tools/convert_dataset/mot2reid.py script may generate different train_80.txt and val_20.txt in different machines.

[codecov]

Codecov Report

Merging #249 (872b58c) into master (8016b90) will decrease coverage by 0.15%.
The diff coverage is 78.94%.

❗ Current head 872b58c differs from pull request most recent head 9841526. Consider uploading reports for the commit 9841526 to get more accurate results

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
- Coverage   68.95%   68.79%   -0.16%     
==========================================
  Files          90       90              
  Lines        4641     4650       +9     
  Branches      896      897       +1     
==========================================
- Hits         3200     3199       -1     
- Misses       1154     1161       +7     
- Partials      287      290       +3     

Flag	Coverage Δ
unittests	68.79% <78.94%> (-0.14%)	⬇️

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Impacted Files	Coverage Δ
mmtrack/datasets/reid_dataset.py	86.23% <50.00%> (-1.62%)	⬇️
mmtrack/models/mot/trackers/tracktor_tracker.py	64.04% <66.66%> (+0.09%)	⬆️
mmtrack/models/mot/trackers/sort_tracker.py	75.23% <100.00%> (+0.48%)	⬆️
mmtrack/models/reid/base_reid.py	88.00% <100.00%> (+1.04%)	⬆️
mmtrack/datasets/sot_train_dataset.py	78.78% <0.00%> (-7.08%)	⬇️

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