fix grid_sampler PADDLE_ENFORCE error. test=develop (#15542)

paddle/fluid/operators/grid_sampler_op.cc

@@ -43,12 +43,14 @@ class GridSampleOp : public framework::OperatorWithKernel {
     PADDLE_ENFORCE(grid_dims[3] == 2, "Input(Grid) dims[3] should be 2.");
     PADDLE_ENFORCE_EQ(grid_dims[0], x_dims[0],
                       "Input(X) and Input(Grid) dims[0] should be equal.");
+    if (ctx->IsRuntime()) {
       PADDLE_ENFORCE_EQ(
           grid_dims[1], x_dims[2],
           "Input(X) dims[2] and Input(Grid) dims[1] should be equal.");
       PADDLE_ENFORCE_EQ(
           grid_dims[2], x_dims[3],
           "Input(X) dims[3] and Input(Grid) dims[2] should be equal.");
+    }
 
     ctx->SetOutputDim("Output", x_dims);
     ctx->ShareLoD("X", "Output");