fix the bug in expand_v2 op (#30984)

* update, test=develop

paddle/fluid/operators/expand_v2_op.cc

@@ -66,6 +66,9 @@ class ExpandV2Op : public framework::OperatorWithKernel {
         out_shape[i] = -1;
       } else if (expand_shape[i] == -1) {
         out_shape[i] = x_dims[i];
+      } else if (expand_shape[i] == -2) {
+        // We use -2 to represent the element in expand_shape is a var.
+        out_shape[i] = -1;
       } else {
         PADDLE_ENFORCE_GT(
             expand_shape[i], 0,
@@ -174,7 +177,7 @@ class ExpandV2GradOp : public framework::OperatorWithKernel {
     x_dim_vec.insert(x_dim_vec.begin(), diff, -1);
 
     for (size_t i = 0; i < expand_shape.size(); ++i) {
-      if (expand_shape[i] == -1 || x_dim_vec[i] == -1) {
+      if (expand_shape[i] < 0 || x_dim_vec[i] == -1) {
         continue;
       } else {
         if (ctx->IsRuntime()) {

python/paddle/tensor/manipulation.py

@@ -1448,7 +1448,7 @@ def expand(x, shape, name=None):
         attrs_expand_shape = []
         for idx, shape in enumerate(list_expand_shape):
             if isinstance(shape, Variable):
-                attrs_expand_shape.append(-1)
+                attrs_expand_shape.append(-2)
             else:
                 attrs_expand_shape.append(shape)
                 assert shape > 0 or shape == -1, (