Add Qlib notebook tutorial (#1037)

* Add Qlib notebook tutorial

* Update tutorial

qlib/data/dataset/__init__.py

@@ -199,6 +199,9 @@ class DatasetH(Dataset):
 
         col_set : str
             The col_set will be passed to self.handler when fetching data.
+            TODO: make it automatic:
+                - select DK_I for test data
+                - select DK_L for training data.
         data_key : str
             The data to fetch:  DK_*
             Default is DK_I, which indicate fetching data for **inference**.