fix typo in to_onnx.py

[bibs2091]

Add size to the last part of the code, otherwise the max_sizes wont be the same resulting in weird boxes positions:

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.55%.

Quality metrics	Before	After	Change
Complexity	1.74 ⭐	1.77 ⭐	0.03 👎
Method Length	99.50 🙂	101.67 🙂	2.17 👎
Working memory	8.84 🙂	8.87 🙂	0.03 👎
Quality	64.35% 🙂	63.80% 🙂	-0.55% 👎

Other metrics	Before	After	Change
Lines	143	151	8

Changed files	Quality Before	Quality After	Quality Change
converters/to_onnx.py	64.35% 🙂	63.80% 🙂	-0.55% 👎

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
converters/to_onnx.py	main	4 ⭐	241 ⛔	11 😞	49.88% 😞	Try splitting into smaller methods. Extract out complex expressions
converters/to_onnx.py	M.forward	0 ⭐	121 😞	7 🙂	72.87% 🙂	Try splitting into smaller methods

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