[Cherry-pick for 0.17.1] Fix convert_bounding_box_format when passing strings

[NicolasHug]

Cherry-pick of #8265

[pytorch-bot]

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⏳ 5 Pending, 1 Unrelated Failure

As of commit a80ee39 with merge base e12d200 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• Tests / unittests-windows (3.10, windows.4xlarge, cpu) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128

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