[Fix] Fix the error of BCE loss when batch size is 1.

[274869388]

Motivation

The motivation of this PR is to fix the error of binary_cross_entropy when the batch size is 1.

Modification

• binary_cross_entropy
  Modify the pred.squeeze() to pred.squeeze(1) to avoid the error caused by squeeze dimension 0 when batch size is 1.

BC-breaking (Optional)

No

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 MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #1629 (d0c9a52) into master (ef90522) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1629   +/-   ##
=======================================
  Coverage   90.25%   90.25%           
=======================================
  Files         142      142           
  Lines        8477     8477           
  Branches     1428     1428           
=======================================
  Hits         7651     7651           
  Misses        586      586           
  Partials      240      240           

Flag	Coverage Δ
unittests	90.25% <100.00%> (ø)

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

Impacted Files	Coverage Δ
mmseg/models/losses/cross_entropy_loss.py	86.90% <100.00%> (ø)

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