[Fix] Fix the memory-leak problem in init_detector

[Tai-Wang]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.
Fix the memory-leak problem in init_detector by always mapping the weights to cpu.

Modification

Please briefly describe what modification is made in this PR.
Follow the similar issue #6405 in mmdetection.

BC-breaking (Optional)

Does the modification introduce changes that break the back-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.
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.
4. The documentation has been modified accordingly, like docstring or example tutorials.