Collect all contrib models in __init__ and add unit tests for init

qlib/contrib/model/linear.py

@@ -3,7 +3,7 @@
 
 import numpy as np
 import pandas as pd
-
+from typing import Text, Union
 from scipy.optimize import nnls
 from sklearn.linear_model import LinearRegression, Ridge, Lasso
 
@@ -84,7 +84,7 @@ class LinearModel(Model):
             self.coef_ = coef
             self.intercept_ = 0.0
 
-    def predict(self, dataset, segment="test"):
+    def predict(self, dataset: DatasetH, segment: Union[Text, slice] = "test"):
         if self.coef_ is None:
             raise ValueError("model is not fitted yet!")
         x_test = dataset.prepare(segment, col_set="feature", data_key=DataHandlerLP.DK_I)