Fix img seg tests (load checkpoints from `hf-internal-testing`)
Author: mishig25
Pull request: yes

# What does this PR do?

Tests use `tiny-detr` checkpoints from `hf-internal-testing` org from hub

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Fixes # (issue)


## Before submitting
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Comments:
_The documentation is not available anymore as the PR was closed or merged._

@sgugger please feel free to merge it 😊 (the merge btn is not available)