Polish reshape error message under @to_static (#40599)

python/paddle/fluid/layers/nn.py

@@ -6299,7 +6299,14 @@ def reshape(x, shape, actual_shape=None, act=None, inplace=False, name=None):
                 if dim_size == -1:
                     assert unk_dim_idx == -1, (
                         "Only one dimension value of 'shape' in reshape can "
-                        "be -1. But received shape[%d] is also -1." % dim_idx)
+                        "be -1. But received shape[%d] is also -1.\n"
+                        "\n\t# N = x.shape()[2]\t\t# N is an int. "
+                        "(NOT recommend under @to_static)\n\tN = paddle.shape(x)[2]\t\t"
+                        "# N is a Tensor. (Recommend)\n\tz = paddle.reshape([N, -1, 4])"
+                        "\t# z.shape is [-1, -1, 4]\n\n"
+                        "    If your target shape in Reshape represents dynamic shape, "
+                        "please turn it into a Tensor under @to_static. See above example for details."
+                        % dim_idx)
                     unk_dim_idx = dim_idx
                 elif dim_size == 0:
                     assert dim_idx < len(x.shape), (