Given a tensor input, this operation inserts a dimension of size 1 at the
dimension index axis of input's shape. The dimension index axis starts
at zero; if you specify a negative number for axis it is counted backward
from the end.
This operation is useful if you want to add a batch dimension to a single
element. For example, if you have a single image of shape [height, width,
channels], you can make it a batch of one image with expand_dims(image, 0),
which will make the shape [1, height, width, channels].