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Summary:
In StreamWriter basic usage tutorial, matplotlib is used to generate raster images of waveforms, and the figure used is left unshown in the resulting tutorial with the use of ``sphinx_gallery_defer_figures`` command.

It turned out that this figure is shown in the next code block executed by Sphinx Gallery, and the figure is placed in totally unrelated place. https://pytorch.org/audio/main/tutorials/audio_feature_extractions_tutorial.html

<img width="951" alt="Screen Shot 2022-10-14 at 10 06 58 PM" src="https://user-images.githubusercontent.com/855818/195855124-ecd9be49-5085-4acd-9a93-608d9d1ee9ce.png">

This commit fixes it by closing the figure.

Pull Request resolved: https://github.com/pytorch/audio/pull/2771

Reviewed By: nateanl

Differential Revision: D40382076

Pulled By: mthrok

fbshipit-source-id: 015f2bab8492d3b4fbe70e1174c7776a5aa2679a