Clean unnecessary custom_imports in entrypoints

[FloydHsiu]

Motivation

According to #6543, MMCV already supports custom_imports since PR open-mmlab/mmcv#606. There is no need to manually handle custom_imports after calling Config.fromfile. So the code like below should be cleaned.

if cfg.get('custom_imports', None): 
     from mmcv.utils import import_modules_from_strings 
     import_modules_from_strings(**cfg['custom_imports']) 

Modification

1. Search all files with related code

List of files with related code:

• mmdet/core/export/pytorch2onnx.py
• tools/analysis_tools/analyze_results.py
• tools/analysis_tools/eval_metric.py
• tools/analysis_tools/get_flops.py
• tools/analysis_tools/test_robustness.py
• tools/misc/browse_dataset.py
• tools/misc/print_config.py
• tools/test.py
• tools/train.py

2. delete the code in the list.

[CLAassistant]

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[codecov]

Codecov Report

Merging #6546 (4821a3d) into master (c76ab0e) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6546   +/-   ##
=======================================
  Coverage   62.04%   62.04%           
=======================================
  Files         319      319           
  Lines       25343    25340    -3     
  Branches     4194     4193    -1     
=======================================
- Hits        15723    15722    -1     
+ Misses       8794     8792    -2     
  Partials      826      826           

Flag	Coverage Δ
unittests	62.01% <ø> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
mmdet/core/export/pytorch2onnx.py	14.81% <ø> (+0.77%)	⬆️
mmdet/core/bbox/samplers/random_sampler.py	75.00% <0.00%> (-5.56%)	⬇️
mmdet/core/bbox/assigners/max_iou_assigner.py	73.68% <0.00%> (+1.31%)	⬆️

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