Make custom solve_triangluar optional

[fariedabuzaid]

The custom implementation of the solve_triangular function is only necessary if onnx export is needed but is a performance bottleneck. We should make it optional to use the implementation in torch.linalg.

• Extend solve_triangular with another argument, e.g. backend.
  • Depending on the value of the new argument, the function should either use the building implementation or call the torch implementation.
  • Set the default backend to torch.