Replaced opencv with imagesize to get the actual image size

[cansik]

As described in https://github.com/RangiLyu/nanodet/issues/530, this PR replaces the use of opencv with the imagesize package to increase performance.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (226d8cc) 74.80% compared to head (b87f72c) 74.79%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #548 +/- ## ========================================== - Coverage 74.80% 74.79% -0.01% ========================================== Files 71 71 Lines 4695 4694 -1 Branches 731 731 ========================================== - Hits 3512 3511 -1 Misses 986 986 Partials 197 197 ``` | [Flag](https://app.codecov.io/gh/RangiLyu/nanodet/pull/548/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/RangiLyu/nanodet/pull/548/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `74.79% <100.00%> (-0.01%)` | :arrow_down: | 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=None#carryforward-flags-in-the-pull-request-comment) to find out more.

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[cansik]

Strange, I did not change anything for the one-stage detector...