Random flip per channel seems redundant

[Nic-Ma]

A question from internal team:

MONAI/monai/transforms/spatial/array.py

Line 285 in bcdee8c

class Flip(Transform):

random flip per channel seems redundant, you can just flip once:
np.flip(img, self.spatial_axis+1)

[Nic-Ma]

I will check several similar transforms next week.

Thanks.