[Fix] Fix a bug when load checkpoints in mulit-GPUs environment.

[mzr1996]

Motivation

Refers to open-mmlab/mmdetection#6405

Modification

When loading a checkpoint, map the checkpoint to the CPU, instead of None in a multi-GPUs environment.

BC-breaking (Optional)

No

Use cases (Optional)

No

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #527 (b3358d6) into master (29f5121) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #527      +/-   ##
==========================================
+ Coverage   79.48%   79.49%   +0.01%     
==========================================
  Files         106      106              
  Lines        5975     5974       -1     
  Branches      968      968              
==========================================
  Hits         4749     4749              
+ Misses       1095     1094       -1     
  Partials      131      131              

Flag	Coverage Δ
unittests	79.49% <0.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/apis/inference.py	20.00% <0.00%> (+0.35%)	⬆️

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[Ezra-Yu]

LGTM.