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] =
 -0.668873  -0.153963
 -0.647854  -0.33044

[:, :, 2] =
  0.27999    0.775449
 -0.204135  -0.527838

[:, :, 3] =
 0.270886  -0.549341
 0.235716  -0.754509

[:, :, 4] =
  0.633116  -0.270553
 -0.69502    0.207155
julia> r4×2×2 Tensor{Float64, 3, Array{Float64, 3}}:
[:, :, 1] =
 -1.21742  -0.982754
  0.0       0.51828
  0.0       0.0
  0.0       0.0

[:, :, 2] =
 -0.984218  -0.597685
 -0.179305   0.135688
 -0.427365  -0.726178
  0.0        0.055193