[Enhancement] Improve registry infer_scope

[zhouzaida]

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Motivation

resolve open-mmlab/mmcv#1116

• Before this PR

>>> from mmengine import Registry
>>> CONVERTERS = Registry('converter')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/mnt/data/codebases/mmengine/mmengine/registry/registry.py", line 250, in __init__
    self._scope = self.infer_scope() if scope is None else scope
  File "/mnt/data/codebases/mmengine/mmengine/registry/registry.py", line 310, in infer_scope
    filename = inspect.getmodule(sys._getframe(2)).__name__  # type: ignore
AttributeError: 'NoneType' object has no attribute '__name__'

• After this PR

>>> from mmengine import Registry
>>> CONVERTERS = Registry('converter')  # work as expected

Modification

• Improve infer_scope at mmengine/registry/registry.py

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.

[codecov]

Codecov Report

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Patch has no changes to coverable lines.

Additional details and impacted files

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  Misses          ?     1838           
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Flag	Coverage Δ
unittests	78.18% <0.00%> (?)

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