reduce tests
julia> A = Tensor(Dense(SparseList(Element(0.0))), [0.0 0.0 4.4; 1.1 0.0 0.0; 2.2 0.0 5.5; 3.3 0.0 0.0])
Dense [:,1:3]
├─[:,1]: SparseList (0.0) [1:4]
│ ├─[2]: 1.1
│ ├─[3]: 2.2
│ ├─[4]: 3.3
├─[:,2]: SparseList (0.0) [1:4]
├─[:,3]: SparseList (0.0) [1:4]
│ ├─[1]: 4.4
│ ├─[3]: 5.5
julia> reduce(+, A, dims = (1,))
Dense [:,1:3]
├─[:,1]: Dense [1:1]
│ ├─[1]: 6.6
├─[:,2]: Dense [1:1]
│ ├─[1]: 0.0
├─[:,3]: Dense [1:1]
│ ├─[1]: 9.9
julia> reduce(+, A, dims = 1)
Dense [:,1:3]
├─[:,1]: Dense [1:1]
│ ├─[1]: 6.6
├─[:,2]: Dense [1:1]
│ ├─[1]: 0.0
├─[:,3]: Dense [1:1]
│ ├─[1]: 9.9
julia> reduce(+, A, dims = (2,))
Dense [:,1:1]
├─[:,1]: SparseHash (0.0) [1:4]
│ ├─[1]: 4.4
│ ├─[2]: 1.1
│ ├─[3]: 7.7
│ ├─[4]: 3.3
julia> reduce(+, A, dims = 2)
Dense [:,1:1]
├─[:,1]: SparseHash (0.0) [1:4]
│ ├─[1]: 4.4
│ ├─[2]: 1.1
│ ├─[3]: 7.7
│ ├─[4]: 3.3
julia> reduce(+, A, dims = (1, 2))
Dense [:,1:1]
├─[:,1]: Dense [1:1]
│ ├─[1]: 16.5
julia> reduce(+, A, dims = (:))
16.5

