Add __repr__ to SerializedDagModel (#9862)
Add __repr__ to SerializedDagModel (#9862)

Before: `<airflow.models.serialized_dag.SerializedDagModel at 0x7fab30d68c50>`
After: `<SerializedDag: example_xcom_args>`
lyft/incubator-airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,sekikn/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,bolkedebruin/airflow,airbnb/airflow,mistercrunch/airflow,danielvdende/incubator-airflow,DinoCow/airflow,cfei18/incubator-airflow,apache/airflow,lyft/incubator-airflow,bolkedebruin/airflow,apache/airflow,Acehaidrey/incubator-airflow,DinoCow/airflow,sekikn/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,mrkm4ntr/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,DinoCow/airflow,apache/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,DinoCow/airflow,airbnb/airflow,lyft/incubator-airflow,mrkm4ntr/incubator-airflow,mrkm4ntr/incubator-airflow,dhuang/incubator-airflow,airbnb/airflow,apache/airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,apache/incubator-airflow,mrkm4ntr/incubator-airflow,apache/airflow,nathanielvarona/airflow,airbnb/airflow,nathanielvarona/airflow