MatrixDiagPart

diagonal[i, j, ..., l, n]
   = input[i, j, ..., l, n+y, n+x] ; if 0 <= n+y < M and 0 <= n+x < N,
     padding_value                 ; otherwise.
 

diagonal[i, j, ..., l, m, n]
   = input[i, j, ..., l, n+y, n+x] ; if 0 <= n+y < M and 0 <= n+x < N,
     padding_value                 ; otherwise.
 

input = np.array([[[1, 2, 3, 4],  # Input shape: (2, 3, 4)
                    [5, 6, 7, 8],
                    [9, 8, 7, 6]],
                   [[5, 4, 3, 2],
                    [1, 2, 3, 4],
                    [5, 6, 7, 8]]])
 
 # A main diagonal from each batch.
 tf.matrix_diag_part(input) ==> [[1, 6, 7],  # Output shape: (2, 3)
                                 [5, 2, 7]]
 
 # A superdiagonal from each batch.
 tf.matrix_diag_part(input, k = 1)
   ==> [[2, 7, 6],  # Output shape: (2, 3)
        [4, 3, 8]]
 
 # A tridiagonal band from each batch.
 tf.matrix_diag_part(input, k = (-1, 1))
   ==> [[[2, 7, 6],  # Output shape: (2, 3, 3)
         [1, 6, 7],
         [5, 8, 0]],
        [[4, 3, 8],
         [5, 2, 7],
         [1, 6, 0]]]
 
 # Padding value = 9
 tf.matrix_diag_part(input, k = (1, 3), padding_value = 9)
   ==> [[[4, 9, 9],  # Output shape: (2, 3, 3)
         [3, 8, 9],
         [2, 7, 6]],
        [[2, 9, 9],
         [3, 4, 9],
         [4, 3, 8]]]