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]: Sparse (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]: Sparse (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

