Fix obj det image size
Author: mishig25
Pull request: yes

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Fixes # (issue)
In object detection pipeline, changed order of 2 operations. 

This PR gets input image size **before** the image goes through feature_extractor (`self.feature_extractor(images=images,`) so that obj detection piepleine output can be in the original width & height.

Otherwise, `feature_extractor` [resizes the image in-place](https://github.com/huggingface/transformers/blob/master/src/transformers/models/detr/feature_extraction_detr.py#L134-L143) and was causing object detection pipeline output to be in resized width & height, not the original width & height.

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Comments:
Superseded by https://github.com/huggingface/transformers/pull/13308/commits/b04d5d34b6c38e35a2fb6a2d3aec31861c300d96