Extend the supported types of decodePNG

[datumbox]

Fixes #2823

This PR adds support of RGB with transparency, grayscale, grayscale with transparency and palette to the decodePNG() method. To test the changes we reuse the test_decode_png() unit-test and we add additional data. The test compares for differences between reading data from PIL vs torchvision.io.image.

The following script was used to produce the additional images required for the test:

from PIL import Image

# manually downloaded from https://pytorch.org/assets/images/pytorch-logo.png
original = 'pytorch-logo.png'
path = 'rgbalpha_pytorch.png'
with Image.open(original) as img:
    img = img.convert("RGBA")
    img = img.resize((100, 100)) 
    img.save(path)

with Image.open(path) as img:
    img = img.convert("P", palette=Image.ADAPTIVE, colors=32) 
    img.save("pallete_pytorch.png")

with Image.open(path) as img:
    img = img.convert("L") 
    img.save("gray_pytorch.png")

with Image.open(path) as img:
    img = img.convert("LA") 
    img.save("grayalpha_pytorch.png")

with Image.open(path) as img:
    img = img.convert("RGB") 
    img.save("rgb_pytorch.png")

Notes:

• To avoid any licensing issues, we use the pytorch logo.
• The images are shrunk to 100x100 pixels to keep their total size less than 4kb.
• The use of real images allows us to reuse existing tests. It also allows for easier manual inspection/visualization.

[fmassa]

Looks great, thanks a lot!