[Refactor] Use the new interface of fileio from mmengine

[zhouzaida]

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Motivation

Use the new interface of fileio from MMEngine (open-mmlab/mmengine#533) which requires mmengine>=0.2.0.

Modification

• Update mmengine>=0.2.0 in requirements/runtime.txt
• Add a new parameter backend_args and remove file_client_args

BC-breaking (Optional)

Yes. This PR removed the file_client_args parameter in several functions or classes.

Use cases (Optional)

Before this PR

img = mmcv.imread(s3_img_path, file_client_args={'backend': 'http'})

After this PR

img = mmcv.imread(http_img_path, backend_args={'backend': 'http'})

The only difference between file_client_args and backend_args is that the former may contain a prefix field, because in the previous fileio (mmengine < 0.2.0), there were multiple file backends with the same prefix, so you need to specify the prefix in file_client_args.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

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