Improved utilites, adds examples, tests

[oke-aditya]

Fixes #3364 Fixes #3449.

Follow up of plotting utils #3330 #2785 #3296

• Add Test to check modifications are not in place.
• Add return type for all.
• Bump codecov of utils.
• Add GitHub gist notebook for all plotting utils. Show a demonstration with torchvision models.
• Add Colab link.
• Add note in Readme.

I'm still not sure how we should handle single-channel images.

[oke-aditya]

Almost there!

Since we modified utils.make_grid() I rewrote docs for make_grid.

I am adding this to the examples folder, not as anonymous gist.

The advantage is that this can be version controlled as well as modified by anyone with a simple PR!

This is still WIP, but open for reviews!

@datumbox I remember getting nice output on persons. See here!

Can we use this image for showing the segmentation masks demonstration? It would be really nice.

Edit: I'm currently using a sample COCO image which was also used in DETR Colab demo. It too looks nice.
But I'm not sure of copyright issues (if we are allowed to use this image).

[oke-aditya]

Feel free to review the PR!

Here is a link to the rendered notebook.

For bounding boxes, let me know if I should change the image to image as used in segmentations.
Maybe the detection output would be better.

The tests just improve code coverage, I have just added corner cases of errors and some lines which weren't tested earlier.

cc @fmassa @pmeier

[fmassa]

Looks great, thanks!

[codecov]

Codecov Report

Merging #3594 (3ba8d21) into master (20a771e) will increase coverage by 0.36%.
The diff coverage is 100.00%.

❗ Current head 3ba8d21 differs from pull request most recent head feb6fd2. Consider uploading reports for the commit feb6fd2 to get more accurate results

@@            Coverage Diff             @@
##           master    #3594      +/-   ##
==========================================
+ Coverage   79.33%   79.69%   +0.36%     
==========================================
  Files         105      105              
  Lines        9822     9822              
  Branches     1582     1582              
==========================================
+ Hits         7792     7828      +36     
+ Misses       1538     1517      -21     
+ Partials      492      477      -15     

Impacted Files	Coverage Δ
torchvision/utils.py	79.03% <100.00%> (+16.93%)	⬆️
torchvision/transforms/functional_tensor.py	79.84% <0.00%> (+0.56%)	⬆️
torchvision/datasets/folder.py	78.57% <0.00%> (+1.02%)	⬆️
torchvision/models/detection/anchor_utils.py	94.66% <0.00%> (+1.33%)	⬆️
torchvision/ops/deform_conv.py	72.30% <0.00%> (+3.07%)	⬆️
torchvision/extension.py	67.21% <0.00%> (+13.11%)	⬆️

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