More dataloader example (#1823)

* More dataloader example

* optimize code

* optimeze code

* optimeze code

* optimeze code

* optimeze code

* optimeze code

* fix pylint error

* fix CI error

* fix CI error

* Comments

* fix error type

---------

Co-authored-by: Young <afe.young@gmail.com>

qlib/data/dataset/loader.py

@@ -41,6 +41,7 @@ class DataLoader(abc.ABC):
         ----------
         instruments : str or dict
             it can either be the market name or the config file of instruments generated by InstrumentProvider.
+            If the value of instruments is None, it means that no filtering is done.
         start_time : str
             start of the time range.
         end_time : str
@@ -50,6 +51,11 @@ class DataLoader(abc.ABC):
         -------
         pd.DataFrame:
             data load from the under layer source
+
+        Raise
+        -----
+        KeyError:
+            if the instruments filter is not supported, raise KeyError
         """
 
 
@@ -320,7 +326,13 @@ class NestedDataLoader(DataLoader):
     def load(self, instruments=None, start_time=None, end_time=None) -> pd.DataFrame:
         df_full = None
         for dl in self.data_loader_l:
-            df_current = dl.load(instruments, start_time, end_time)
+            try:
+                df_current = dl.load(instruments, start_time, end_time)
+            except KeyError:
+                warnings.warn(
+                    "If the value of `instruments` cannot be processed, it will set instruments to None to get all the data."
+                )
+                df_current = dl.load(instruments=None, start_time=start_time, end_time=end_time)
             if df_full is None:
                 df_full = df_current
             else: