Update docstings and expose main image io functions in the vision docs

[andfoy]

This PR exposes the main encoding/reading and decoding/writing functions for the image backend:

• read_image
• decode_image
• encode_jpeg
• write_jpeg
• encode_png
• write_png

[codecov]

Codecov Report

Merging #2760 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2760   +/-   ##
=======================================
  Coverage   73.09%   73.09%           
=======================================
  Files          96       96           
  Lines        8321     8322    +1     
  Branches     1295     1295           
=======================================
+ Hits         6082     6083    +1     
  Misses       1840     1840           
  Partials      399      399           

Impacted Files	Coverage Δ
torchvision/io/image.py	80.00% <ø> (ø)
torchvision/io/__init__.py	100.00% <100.00%> (ø)

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[fmassa]

@andfoy thanks for the PR!

FYI we will be removing read_jpeg and read_png before the release (and instead let the user use read_image only).

I'll be sending a follow-up PR removing those

[fmassa]

Good to go, but I'll be removing read_png and read_jpeg in a follow-up PR

[andfoy]

Should we do something similar for the encode function? Let users to pass a format argument?

[fmassa]

Should we do something similar for the encode function? Let users to pass a format argument?

I'm ok for now letting it be explicit via separate functions, otherwise we would need to do some more extra work just before the release (different kwargs for different functions, and kwargs don't work with torchscript).

[fmassa]

I would though consider factoring out the saving part from write_png / write_jpeg so that we have a custom function just for writing a file, similar to read_file but for writing, a la write_file.

[fmassa]

FYI I'm working on implementing a write_file.