[Fix] Fix install from source issue where get_version raises KeyError

[nil0x9]

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Motivation

Currently when installing mmengine from source, one might encounter such a error:

  from pkg_resources import DistributionNotFound, get_distribution
Traceback (most recent call last):
  File "/.../codebase/mmengine/setup.py", line 118, in <module>
    version=get_version(),
            ~~~~~~~~~~~^^
  File "/.../codebase/mmengine/setup.py", line 32, in get_version
    return locals()['__version__']
           ~~~~~~~~^^^^^^^^^^^^^^^
KeyError: '__version__'

Modification

Modified get_version function in setup.py

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 MMDetection or MMPretrain.
4. The documentation has been modified accordingly, like docstring or example tutorials.

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