Syntactic sugar: sio.Video(filename)

[talmo]

Right now, creating a video with sio.Video(filename) will not initialize the backend, which might lead to weird downstream errors like:

NotImplementedError: Cannot serialize video backend for video: Video(filename="/home/jovyan/talmolab-smb/datasets/mot/animal/sleap/SLAP_M74/four_mice/val/top-10072022151549-0000_h265_CRF12_denoised.mp4", shape=None, backend=NoneType)

We could handle backend initialization (like in sio.Video.from_filename constructor) in the post init similarly to how we handle the sio.Labels(lfs) syntax here:

sleap-io/sleap_io/model/labels.py

Lines 58 to 69 in 45941db

	def __attrs_post_init__(self):
	"""Append videos, skeletons, and tracks seen in `labeled_frames` to `Labels`."""
	for lf in self.labeled_frames:
	if lf.video not in self.videos:
	self.videos.append(lf.video)
	for inst in lf:
	if inst.skeleton not in self.skeletons:
	self.skeletons.append(inst.skeleton)
	if inst.track is not None and inst.track not in self.tracks:
	self.tracks.append(inst.track)

It might also be good to refactor both of these into an instance method in case we want to do these manually.