Set argument image_size configurable in export_model.py

[Yogurt-Peng]

Use the following to set the image size of the ONNX model:

python export_model.py --checkpoint_path [path/to/your/model.pt] --image_size 320 320

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #250 (eb7241d) into main (5dd25c3) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##             main     #250   +/-   ##
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  Coverage   93.62%   93.62%           
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  Files          11       11           
  Lines         675      675           
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  Hits          632      632           
  Misses         43       43           

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unittests	93.62% <ø> (ø)

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[zhiqwang]

LGTM, Thanks @2428513107 for your contribution!