[Fix]: Remove sampling hardcode.

[RangiLyu]

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Motivation

Remove hardcode of determining sampling by loss type.

Resolve #5717 [Bug]: Can not using custom loss when determine sampling by loss type.

Modification

Sampling or not is determined by sampler in train_cfg now.

If there is no sampler or setting PseudoSampler in train_cfg, sampling will be False, otherwise, be True.

And to avoid BC-breaking, when sampler is set in train_cfg, we still keep the loss type logic and raise a warning. These codes will be removed after several versions.

BC-breaking (Optional)

Yes. sampling will not be determined by loss type in the future.

Need to check backward compatibility.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #6317 (caf3b50) into master (c88509c) will increase coverage by 0.01%.
The diff coverage is 50.00%.

❗ Current head caf3b50 differs from pull request most recent head d370a52. Consider uploading reports for the commit d370a52 to get more accurate results

@@            Coverage Diff             @@
##           master    #6317      +/-   ##
==========================================
+ Coverage   61.80%   61.81%   +0.01%     
==========================================
  Files         315      315              
  Lines       25157    25172      +15     
  Branches     4185     4186       +1     
==========================================
+ Hits        15549    15561      +12     
- Misses       8798     8800       +2     
- Partials      810      811       +1     

Flag	Coverage Δ
unittests	61.79% <50.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmdet/models/dense_heads/anchor_head.py	85.34% <50.00%> (-1.39%)	⬇️
mmdet/models/detectors/two_stage.py	68.23% <0.00%> (ø)
mmdet/models/roi_heads/standard_roi_head.py	62.11% <0.00%> (ø)
mmdet/datasets/pipelines/auto_augment.py	90.64% <0.00%> (+0.07%)	⬆️
mmdet/models/dense_heads/yolact_head.py	79.55% <0.00%> (+0.17%)	⬆️
mmdet/models/detectors/yolact.py	97.36% <0.00%> (+3.08%)	⬆️

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[ZwwWayne]

May also update docs https://mmdetection.readthedocs.io/en/latest/tutorials/customize_losses.html.

[jshilong]

LGTM