[BUG] `fusedims` sometimes initializes all blocks

[ogauthe]

using BlockSparseArrays: BlockSparseArray, eachblockstoredindex
using GradedUnitRanges: gradedrange
using SymmetrySectors: U1
using TensorAlgebra: fusedims

r = gradedrange([U1(0) => 2, U1(1) => 2])
a2 = BlockSparseArray{Float64}(undef, r, r)
@test length(eachblockstoredindex(fusedims(a2, (1,), (2,)))) == 0
a3 = BlockSparseArray{Float64}(undef, r, r, r)
@test_broken length(eachblockstoredindex(fusedims(a3, (1,), (2,), (3,)))) == 0

For some reason this appears starting at 3 dimensions. Unaffected by current rewriting using unmerged_tensor_product.

[mtfishman]

Huh, that's strange, thanks for the report.

[ogauthe]

The new matricize does not suffer from this issue.