check if view is group before assigning slice

[sashankaryal]

This PR makes sure that for grouped datasets with videos, if SelectGroupSlices view stage is applied with only one slice, the resulting view is only assigned group_slice if it's media type is fom.GROUP.

import fiftyone as fo
import fiftyone.zoo as foz

dataset = fo.Dataset("video-slice-bug")

# qsv = fo.load_dataset("quickstart-video")
qsv = foz.load_zoo_dataset("quickstart-video", max_samples=1)

group = fo.Group()
sample = fo.Sample(qsv.first().filepath, group=group.element("video"))
dataset.add_sample(sample)

view = dataset.select_group_slices(["video"])
fo.launch_app(view)

[codecov]

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##             release/v0.23.5    #4066   +/-   ##
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[benjaminpkane]

LGTM ✅