Off-by-one issues with the frame sampling

[bryant1410]

I think there may be 2 off-by-one issues with the frame sampling. I'm not so sure about it and prefer to discuss it, that's why I don't send a patch.

For the first one, this is the part of the code:

frozen-in-time/base/base_dataset.py

Lines 152 to 155 in 542164b

	intervals = np.linspace(start=0, stop=vlen, num=acc_samples + 1).astype(int)
	ranges = []
	for idx, interv in enumerate(intervals[:-1]):
	ranges.append((interv, intervals[idx + 1] - 1))

I think it should be:

np.linspace(start=0, stop=vlen - 1, ...)

(with a - 1)

and:

ranges.append((interv, intervals[idx + 1]))

(without the - 1).

Otherwise, the right part of each bucket it's gonna be ignored. For the uniform case, instead of doing the interval centroid ((a+b)/2), it takes (a+b-1)/2. This isn't a big deal though.

For the second one, I think the random choice interval end should be + 1. When it does random.choice(range(...)) (which btw could be a random.randrange), the range excludes the stop value, so there's another - 1 hidden there.

For example, in the training video "1013731484", which has only one frame according to Decord, for random it'd be:

intervals == [0, 1]
ranges = [(0, 0)]
random.choice(range(0, 0)) == random.choice([]) <- exception

And it fails silently, assigning all frames to black. Note this one also isn't a big deal as it'd fail with few videos, and with the rest, it'd have all the intervals shifted or something like that.