What does SpatialDropout1D() do to output of Embedding() in Keras?
Basically, it removes all the pixel in a row from all channels. 
eg: take [[1,1,1], [2,4,5]], there are 3 points with values in 2 channels, by doing SpatialDropout1D it zeros an entire row ie all attributes of a point is set to 0; like [[1,1,0], [2,4,0]]

number of such choices would be 3C0 + 3C1+ 3C2 + 3C3 = 8

The intuition behind this is in many cases for an image the adjacent pixels are correlated, so hiding one of them is not helping much, rather hiding entire row, that's gotta make a difference. reference material