[FR] Decouple loading video samples from their frames?

[brimoor]

Originally posted by @brimoor in #858 (comment)

I'd like to find a clean way to decouple loading the Frames of a video Sample from loading the sample itself. Currently we have this behavior:

import fiftyone.zoo as foz

dataset = foz.load_zoo_dataset("quickstart-video")

# takes a non-trivial amount of time since the frames are always 
# attached, even if I don't want them
print(dataset.first())

I'd rather make frame loading JIT if possible (noting that, when DatasetViews are involved, the aggregation pipeline may have internally attached and then detached frames in order to compute which samples are in the view):

for sample in video_dataset:
    # `sample` loads fast because `frames` haven't been loaded yet
    print(sample.id)
    
    # now frames are loaded
    print(len(sample.frames))