Add new illustrations of transforms

With https://github.com/pytorch/vision/pull/3652 merged, we can now illustrate how each transforms affects an image via a sphinx-gallery example:

https://519065-73328905-gh.circle-artifacts.com/0/docs/auto_examples/plot_transforms.html#sphx-glr-auto-examples-plot-transforms-py

The goal of this issue is to add more transforms to this file: https://github.com/pytorch/vision/blob/master/gallery/plot_transforms.py

Instructions:

• pick you favourite transform that isn't already present in the example (after checking below that no one is already working on it)
• comment below "I'm working on <the transform>" so that others don't pick the same one
• edit the example file with something that properly illustrate the transform, taking inspiration from the already present transforms
• Build the docs following the instructions here, check that everything is fine, then submit a PR.

You can pick more than one transform in the same PR if you feel like it.

cc @vfdev-5

Start with gallery/plot_transforms.py and compare the existing transform illustrations, then read the documentation build instructions in CONTRIBUTING.md. Choose a transform not already shown, add an appropriate illustration to the example, build the docs, and confirm the generated example renders correctly before submitting a PR.