Add training code for gencast

Author: liaoxin2

docs/zh/examples/gencast.md

@@ -10,7 +10,7 @@
     ``` sh
     # 设置路径到 PaddleScience/jointContribution 文件夹
     cd PaddleScience/jointContribution
-    export PYTHONPATH=$PWD:$PYTHONPAT
+    export PYTHONPATH=$PWD:$PYTHONPATH
     # 运行训练脚本
     python run_gencast.py mode=train
     ```
@@ -20,7 +20,7 @@
     ``` sh
     # 设置路径到 PaddleScience/jointContribution 文件夹
     cd PaddleScience/jointContribution
-    export PYTHONPATH=$PWD:$PYTHONPAT
+    export PYTHONPATH=$PWD:$PYTHONPATH
     # 下载模型参数
     cd gencast/
     wget -nc https://paddle-org.bj.bcebos.com/paddlescience/models/gencast/gencast_params_GenCast-1p0deg-Mini-_2019.pdparams -P ./data/params/

jointContribution/gencast/run_gencast.py

@@ -28,7 +28,7 @@
 
 class CustomDataLoader(paddle.io.Dataset):
     def __init__(self, target_lead_times, cfg):
-        super(CustomDataLoader, self).__init__()
+        super().__init__()
 
         self.target_lead_times = target_lead_times
         self.cfg = cfg