Fix infer_shape in pad2d_op (#16911)

test=develop

paddle/fluid/operators/pad2d_op.cc

@@ -480,8 +480,10 @@ class Pad2dOp : public framework::OperatorWithKernel {
       PADDLE_ENFORCE_EQ(
           paddings_dim.size(), 1,
           "Size of Input(Paddings)'s dimension should be equal to 1.");
+      if (ctx->IsRuntime()) {
         PADDLE_ENFORCE_EQ(paddings_dim[0], 4,
                           "Shape of Input(Paddings) should be equal to [4].");
+      }
       out_dims[1] = x_dim[1];
       out_dims[2] = x_dim[2];
       out_dims[3] = x_dim[3];
@@ -501,12 +503,8 @@ class Pad2dOp : public framework::OperatorWithKernel {
     }
 
     ctx->SetOutputDim("Out", framework::make_ddim(out_dims));
-    if (out_dims[0] == x_dim[0]) {
-      // Only pass LoD when the first dimension is equal between
-      // output and input.
     ctx->ShareLoD("X", /*->*/ "Out");
   }
-  }
 
  protected:
   framework::OpKernelType GetExpectedKernelType(