[unify] change train_detector to train_model

[EasonQYS]

change function name train_detector to train_model, same as other mmlab api style.

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Motivation

To be same as other mmlab api style.

Modification

change function name train_detector to train_model, same as other mmlab api style.

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.

[CLAassistant]

All committers have signed the CLA.

[hhaAndroid]

@EasonQYS Thank you for your submission! Please sign CLA at first.

[EasonQYS]

Thanks for reminding me.

[codecov[bot]]

Codecov Report

Patch coverage is 1.85% of modified lines.

Files Changed	Coverage
mmdet/apis/train.py	1.85%

:loudspeaker: Thoughts on this report? Let us know!.