Change strict_load of EMAHook to False by default

[HAOCHENYE]

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Motivation

Resolve #607.

Since #542 makes load_state_dict not save or load non-persistent buffer anymore, resuming the old checkpoint with std and mean will cause the error described in #607.

The default resume strategy of EMAHook should be consistent with Runner. Since Runner does not load checkpoint strictly, the default value of load_strict in EMAHook should be False.

Modification

Please briefly describe what modification is made in this PR.

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.