Fix non-distributed training of cuda device specifying

[Youth-Got]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

修复多卡环境下的单卡训练的cuda设备id设置无效的bug。

Modification

mmdet/utrils/util_distribution.py

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

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 #8176 (2e0a025) into dev (240d7a3) will decrease coverage by 0.52%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev    #8176      +/-   ##
==========================================
- Coverage   64.69%   64.17%   -0.53%     
==========================================
  Files         351      361      +10     
  Lines       28463    29525    +1062     
  Branches     4807     5019     +212     
==========================================
+ Hits        18414    18947     +533     
- Misses       9057     9575     +518     
- Partials      992     1003      +11     

Flag	Coverage Δ
unittests	64.14% <ø> (-0.55%)	⬇️

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

Impacted Files	Coverage Δ
mmdet/apis/train.py	13.68% <ø> (ø)
mmdet/core/__init__.py	100.00% <ø> (ø)
mmdet/core/bbox/assigners/atss_assigner.py	92.04% <ø> (-1.47%)	⬇️
mmdet/core/evaluation/eval_hooks.py	82.71% <ø> (+0.89%)	⬆️
mmdet/core/evaluation/mean_ap.py	70.08% <ø> (-1.11%)	⬇️
mmdet/core/hook/__init__.py	100.00% <ø> (ø)
mmdet/core/hook/memory_profiler_hook.py	95.65% <ø> (ø)
mmdet/core/hook/wandblogger_hook.py	15.04% <ø> (ø)
mmdet/core/optimizers/__init__.py	100.00% <ø> (ø)
mmdet/core/optimizers/builder.py	33.33% <ø> (ø)
... and 57 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 240d7a3...2e0a025. Read the comment docs.