Add dataloader

[borisdayma]

Create a dataloader based on this suggested notebook:

• it uses tfrecord for efficiency on TPU (in particular wrt multiprocessing)
• data is downloaded using img2dataset (based on my ongoing PR branch)
• format is webp which seems very advantageous (see my benchmark below)

[borisdayma]

Related to webp format, here is relevant data I'm adding to img2dataset README:

[borisdayma]

This is now supported