Refactor: reduce test warnings

[liyier90]

Addresses https://github.com/aisingapore/PeekingDuck-Private/issues/113

• use float instead of np.float
• use torch.testing.assert_close instead of torch.testing.assert_allclose
• bump sckit-image==0.18.0 and use skimage.metrics.structural_similarity
• use post_process_object_detection instead of post_process in huggingface_hub