Fix bounding_box.converters.convert_format

[james77777778]

What does this PR do?

Fixes #1572

This PR fixes the bug in unit tests as well as _image_shape in converters.

A new unit test called test_dense_bounding_box_with_ragged_images has been added to validate inputs with ragged images and dense bounding_boxes, which were causing error as mentioned in #1537 (comment)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue? Please add a link
  to it if that's the case.
• Did you write any new necessary tests?
• If this adds a new model, can you run a few training steps on TPU in Colab to ensure that no XLA incompatible OP are used?

Who can review?

@LukeWood @ianstenbit

[LukeWood]

Thanks for the fix @james77777778

[LukeWood]

/gcbrun