QR decompostion

julia> t = Tensor(rand(2,2,2,2), [:i,:j,:k,:l]);
julia> q, r = tensor_qr_thin(
           t;
           inds_q = Index.([:i, :k]),
           inds_r = Index.([:j, :l]),
           ind_virtual = Index(:x)
       );
julia> q2×2×4 Tensor{Float64, 3, Array{Float64, 3}}:
[:, :, 1] =
 -0.0642326  -0.451842
 -0.664602   -0.591622

[:, :, 2] =
 -0.496444  -0.123538
 -0.493912   0.703088

[:, :, 3] =
  0.784451  -0.465628
 -0.110432   0.394502

[:, :, 4] =
 -0.366134  -0.750845
  0.54969   -0.00430077
julia> r4×2×2 Tensor{Float64, 3, Array{Float64, 3}}:
[:, :, 1] =
 -1.36281  -0.375719
  0.0      -0.0888963
  0.0       0.0
  0.0       0.0

[:, :, 2] =
 -0.546235  -1.08848
 -0.515815  -0.0187884
  0.289563   0.231799
  0.0        0.257005