fix p_norm with empty shape (#29500)

fix p_norm with empty shape (#29500)

paddle/fluid/operators/p_norm_op.cc

@@ -116,6 +116,9 @@ class PnormOp : public framework::OperatorWithKernel {
       for (int i = 0; i < x_dim.size(); ++i) {
         if (i != axis) reduce_dims.emplace_back(x_dim[i]);
       }
+      if (reduce_dims.size() == 0) {
+        reduce_dims.emplace_back(1);
+      }
     }
     x_dim[axis] = 1;