Add docstrings for input type in YOLOTransform

[zhiqwang]

cc @mattpopovich , Let the GH Actions tell us what happened there.

[zhiqwang]

Hi @mattpopovich ,

First check the test summary here:
https://github.com/zhiqwang/yolov5-rt-stack/runs/5139305768?check_suite_focus=true#step:11:1336

It is mainly used in two scenarios:

• For the dataloader in training and evaluation, the images will be a 4-dim Tensor in that case, and then this operation will bring images back to a list.
  https://github.com/zhiqwang/yolov5-rt-stack/blob/720cd32334bb248ffdca5faf2f014fe04407bafa/yolort/data/_helper.py#L115

• For shape inference when exporting ONNX models. The shape inference mechanism in jit.trace is a little weak, and seems that this line is essential to make it works.

[mattpopovich]

Brilliant! Thank you!

[codecov]

Codecov Report

Merging #308 (6a0d0b6) into main (c4fbbe1) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##             main     #308   +/-   ##
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  Coverage   94.93%   94.93%           
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  Files          11       11           
  Lines         731      731           
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  Hits          694      694           
  Misses         37       37           

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