Improve unittest coverage and minor fix

[dreamerlin]

• Add unittest for inference code to improve unittest coverage.
• Covert #! /usr/bin/bash env to #!/usr/bin/env bash in data preparation scripts.

[codecov]

Codecov Report

Merging #62 into master will increase coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   84.67%   84.90%   +0.23%     
==========================================
  Files          73       73              
  Lines        3889     3889              
  Branches      632      632              
==========================================
+ Hits         3293     3302       +9     
+ Misses        490      485       -5     
+ Partials      106      102       -4     

Flag	Coverage Δ
#unittests	84.90% <ø> (+0.23%)	⬆️

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Impacted Files	Coverage Δ
mmaction/apis/inference.py	83.01% <ø> (+16.98%)	⬆️

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