Tests for MaskFormerFeatureExtractor's post_process*** methods (#15929)
* proper tests for post_process*** methods in feature extractor
* mask th == 0
* Update tests/maskformer/test_feature_extraction_maskformer.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* make style
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>