Matrix sum along a dimension is defined to return a MathScalarType, but might return higher dimension #3202
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Description
Describe the bug
When summing over an axis of a matrix, we always lose just one dimension. If the matrix is 2D, we'll get a 1D array. If the matrix is 3D, we'll get a 2D matrix.
What happens now, is that math.sum declares that it returns a MathScalarType, instead of optionally returning a matrix.
To Reproduce
const mat = math.matrix([[1,2,3],[4,5,6]])
console.log(math.sum(mat, 0))
> DenseMatrix { _data: [ 5, 7, 9 ], _size: [ 3 ], _datatype: undefined }