[Fix] Correctly saves _metadata of state_dict when saving checkpoints

[Bomsw]

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Motivation

Fix #1128, where statde_dict._metadata of saved checkpoints becomes empty OrderedDicts

Modification

modified weights_to_cpu() in runner/checkpoint.py that it stashes _metadata of state_dicts before moving weights to cpu

BC-breaking (Optional)

N/A

Use cases (Optional)

N/A

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.

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[OpenMMLab-Assistant-004]

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