Bump version to v0.7.3

[Z-Fran]

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Modification

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Checklist

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• [ ] Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
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[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (995a5e9) 74.81% compared to head (d297e6d) 74.81%.

:exclamation: Current head d297e6d differs from pull request most recent head 7982da7. Consider uploading reports for the commit 7982da7 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #506 +/- ## ======================================= Coverage 74.81% 74.81% ======================================= Files 145 145 Lines 11015 11015 Branches 2097 2097 ======================================= Hits 8241 8241 Misses 2233 2233 Partials 541 541 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `74.81% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-mmlab#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/open-mmlab/mmgeneration/pull/506?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-mmlab) | Coverage Δ | | |---|---|---| | [mmgen/version.py](https://codecov.io/gh/open-mmlab/mmgeneration/pull/506?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-mmlab#diff-bW1nZW4vdmVyc2lvbi5weQ==) | `58.33% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-mmlab). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-mmlab)

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