Fix a bug and document RandomZoomOut

[datumbox]

Ported from pytorch/vision#5278

We found a bug on the original implementation of RandomZoomOut from TorchVision, so I thought to patch this copy as well. The good news is that we plan soon to move these transforms in TorchVision main library (using the new Transforms API) so you won't have to maintain them in your repo. But until then, this will do.

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[codecov]

Codecov Report

Merging #289 (775bd95) into main (51f9d41) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #289   +/-   ##
=======================================
  Coverage   94.01%   94.01%           
=======================================
  Files          11       11           
  Lines         718      718           
=======================================
  Hits          675      675           
  Misses         43       43           

Flag	Coverage Δ
unittests	94.01% <ø> (ø)

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[zhiqwang]

Thanks @datumbox , LGTM