fix image format
fix image format

Summary: Error met when using test time augmentation in P138569013. Fix the problem by using `image_format` from `tta_mapper`

Reviewed By: ppwwyyxx

Differential Revision: D23079314

fbshipit-source-id: 2931a5cba0f6f26b37eddafaf7dfd03936559d3e

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