Nested data loader (#1822)

* nested data loader

* Amend

* add data loder test

* fix pylint error

* fix pytest error

* fix pytest error

* delete comments

* Update qlib/contrib/data/handler.py

---------

Co-authored-by: Linlang <Lv.Linlang@hotmail.com>

tests/data_mid_layer_tests/test_dataloader.py

@@ -0,0 +1,50 @@
+# TODO:
+# dump alpha 360 to dataframe and merge it with Alpha158
+
+import sys
+import unittest
+import qlib
+from pathlib import Path
+
+sys.path.append(str(Path(__file__).resolve().parent))
+from qlib.data.dataset.loader import NestedDataLoader
+from qlib.contrib.data.loader import Alpha158DL, Alpha360DL
+
+
+class TestDataLoader(unittest.TestCase):
+
+    def test_nested_data_loader(self):
+        qlib.init()
+        nd = NestedDataLoader(
+            dataloader_l=[
+                {
+                    "class": "qlib.contrib.data.loader.Alpha158DL",
+                },
+                {
+                    "class": "qlib.contrib.data.loader.Alpha360DL",
+                    "kwargs": {"config": {"label": (["Ref($close, -2)/Ref($close, -1) - 1"], ["LABEL0"])}},
+                },
+            ]
+        )
+        # Of course you can use StaticDataLoader
+
+        dataset = nd.load()
+
+        assert dataset is not None
+
+        columns = dataset.columns.tolist()
+        columns_list = [tup[1] for tup in columns]
+
+        for col in Alpha158DL.get_feature_config()[1]:
+            assert col in columns_list
+
+        for col in Alpha360DL.get_feature_config()[1]:
+            assert col in columns_list
+
+        assert "LABEL0" in columns_list
+
+        # Then you can use it wth DataHandler;
+
+
+if __name__ == "__main__":
+    unittest.main()