fix writing flow map bug in test

[MeowZheng]

fix #74

[codecov]

Codecov Report

Merging #83 (42a6751) into master (573d79c) will decrease coverage by 0.28%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
- Coverage   68.08%   67.79%   -0.29%     
==========================================
  Files          86       86              
  Lines        4888     4888              
  Branches      704      704              
==========================================
- Hits         3328     3314      -14     
- Misses       1453     1467      +14     
  Partials      107      107              

Flag	Coverage Δ
unittests	67.79% <0.00%> (-0.29%)	⬇️

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Impacted Files	Coverage Δ
mmflow/apis/test.py	16.34% <0.00%> (ø)
mmflow/datasets/pipelines/transforms.py	77.50% <0.00%> (-2.34%)	⬇️

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