Docs for new features merged from samples_on_demand

[philipp-fischer]

Adds docs for MR #111

Notes:
Decoding

• New way to specify media decoding (now SampleDecoder class)
  • Old args auto_decode, image_decode, av_decode, video_decode_audio are now deprecated

class MyTaskEncoder(DefaultTaskEncoder):
    decoder = SampleDecoder(image_decode="pilrgb")

TaskEncoder

• New methods preencode_sample and postencode_sample (optional replacement for encode_sample)

FileStore

• New way to randomly access a prepared dataset using keys
• Datasets can be used as auxiliary file stores

CachePool

• Cache pools can be used to lazily fetch data in the background
• This is useful for online packing where we know we will need say 8k samples in the future, but we don't know the order yet

Cookers

• New cooker definition (in TE)
• New cooker decorator args (need_cache, need_primary)
• New (image/video-) decoder specification in TE directly

[voegtlel]

First comments for crude

[voegtlel]

More comments