Eagerly noop single block_diag with single input

Author: jessegrabowski

pytensor/tensor/slinalg.py

@@ -1402,6 +1402,9 @@ def block_diag(*matrices: TensorVariable):
                      [0, 0, 5, 6],
                      [0, 0, 7, 8]])
     """
+    if len(matrices) == 1:
+        return matrices[0]
+
     _block_diagonal_matrix = Blockwise(BlockDiagonal(n_inputs=len(matrices)))
     return _block_diagonal_matrix(*matrices)