[BUG] Contracting `diag` objects with a non common axes bugs

[JoeyT1994]

The following

using DiagonalArrays: delta

using TensorAlgebra: contract

contract(delta(2, 2), ("i", "j"), delta(2, 2), ("j", "l"))

Errors with

unless the delta arrays have exactly matching indices. Obviously this makes sense from the perspective that the array would no longer be diagonal, but presumably the type should be promoted up to a sparse or dense array in such cases.

[mtfishman]

Since the inputs you show have a single matching index, that maps to a DiagonalMatrix * DiagonalMatrix multiplication, so that should be easy to support since the result is a DiagonalMatrix. Outer products will be harder to support:

contract(delta(2, 2), ("i", "j"), delta(2, 2), ("k", "l"))

since the output is a general sparse array.