[Fix] backend_args should not be modified by get_file_backend

[zhouzaida]

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Motivation

>>> import mmengine.fileio as fileio
>>> backend_args = {'backend': 'local'}
>>> res = fileio.get(filepath, backend_args=backend_args)
>>> backend_args
{}

backend key in backend_args will be popped and it will cause the program to fail when users use it again.

Modification

copy backend_args before popping it.

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

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Patch has no changes to coverable lines.

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Additional details and impacted files

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Flag	Coverage Δ
unittests	78.22% <0.00%> (?)

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