Make infer shape of pad2d support for input with negative dims in compile time. (#18695)

test=develop

paddle/fluid/operators/pad2d_op.cc

@@ -495,13 +495,21 @@ class Pad2dOp : public framework::OperatorWithKernel {
       PADDLE_ENFORCE_EQ(paddings.size(), 4,
                         "Size of paddings should be equal to 4.");
       if (data_format == "NCHW") {
-        out_dims[1] = x_dim[1];
-        out_dims[2] = x_dim[2] + paddings[0] + paddings[1];  // height
-        out_dims[3] = x_dim[3] + paddings[2] + paddings[3];  // width
-      } else {                                               // NHWC
-        out_dims[3] = x_dim[3];
-        out_dims[1] = x_dim[1] + paddings[0] + paddings[1];
-        out_dims[2] = x_dim[2] + paddings[2] + paddings[3];
+        out_dims[1] = x_dim[1];  // channel
+        out_dims[2] = ((!ctx->IsRuntime()) && (x_dim[2] < 0))
+                          ? x_dim[2]
+                          : (x_dim[2] + paddings[0] + paddings[1]);  // height
+        out_dims[3] = ((!ctx->IsRuntime()) && (x_dim[3] < 0))
+                          ? x_dim[3]
+                          : (x_dim[3] + paddings[2] + paddings[3]);  // width
+      } else {                                                       // NHWC
+        out_dims[3] = x_dim[3];                                      // channel
+        out_dims[1] = ((!ctx->IsRuntime()) && (x_dim[1] < 0))
+                          ? x_dim[1]
+                          : (x_dim[1] + paddings[0] + paddings[1]);  // height
+        out_dims[2] = ((!ctx->IsRuntime()) && (x_dim[2] < 0))
+                          ? x_dim[2]
+                          : (x_dim[2] + paddings[2] + paddings[3]);  // width
       }
     }