Fix RTMDet PTQ performance

[sungchul2]

Summary

ticket no. : 144647
There is RTMDet's performance degradation after PTQ.
By updating preset to mixed, the optimized model showed similar performance with the exported model.

PTQ configuration	pothole_small_1	pothole_small_2	pothole_small_3	pothole_medium	vitens_large
export	0.450	0.518	0.500	0.725	0.756
default	0.427 (-0.023)	0.439 (-0.079)	0.433 (-0.067)	0.654 (-0.071)	0.711 (-0.045)
+ preset="mixed"	0.433 (-0.017)	0.521 (+0.003)	0.515 (+0.015)	0.715 (-0.010)	0.753 (-0.003)

How to test

Checklist

• I have added unit tests to cover my changes.​
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  Feel free to contact the maintainers if that's a concern.
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# Copyright (C) 2024 Intel Corporation
# SPDX-License-Identifier: Apache-2.0