'Generate optree for converting `indices` to a onehot representation.
Arguments:
indices (Tensor): Elements must be of numpy integer type for gpu
onehot to work.
axis (int): the axis along the feature length dimension
out (Tensor, optional): where the result will be stored. If out is
None, only the op-tree will be returned.
Returns:
OpTreeNode: the resulting op-tree'

@abc.abstractmethod
def onehot(self, indices, axis, out=None):

    raise NotImplementedError()
