add 'area' from input_dict into output_dict

Author: HydrogenSulfate

ppsci/utils/expression.py

@@ -65,6 +65,10 @@ def train_forward(
                 else:
                     raise TypeError(f"expr type({type(expr)}) is invalid")
 
+            # put field 'area' into output_dict
+            if "area" in input_dicts[i]:
+                output_dict["area"] = input_dicts[i]["area"]
+
             output_dicts.append(output_dict)
 
             # clear differentiation cache