[Fix] runner's config is overwritten by resume()

[SuTanTank]

Motivation

Resolves #1875

Checklist

Before PR:

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

After PR:

• [x] 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.
• [x] CLA has been signed and all committers have signed the CLA in this PR.

[HAOCHENYE]

Thanks for your contribution! It seems that the previous implementation has some bugs. But self.meta = checkpoint['meta'] still seems weird, the current exp_name and env_info will be overwritten, is this reasonable? Maybe @hhaAndroid could have a look.

[SuTanTank]

I'm doing fp16 training recently and maybe I can fix the overwritten issue (introduced by #1013 ) in a more elegant way. No need to merge yet. @HAOCHENYE

Update: new commit pushed.

[SuTanTank]

Any ideas about the CI failure?