SARIMAX -> new arch + fixes

etna/datasets/tsdataset.py

@@ -35,7 +35,7 @@ def __init__(self, df: pd.DataFrame, freq: str, df_exog: Optional[pd.DataFrame]
             frequency of timestamp in df
         df_exog:
             dataframe with exogenous data;
-            if the series is known in the future features' names should start wuth prefix 'regressor_`.
+            if the series is known in the future features' names should start with prefix 'regressor_`.
         """
         self.raw_df = df.copy(deep=True)
         self.raw_df.index = pd.to_datetime(self.raw_df.index)

etna/models/__init__.py

@@ -7,4 +7,5 @@
 from etna.models.moving_average import MovingAverageModel
 from etna.models.naive import NaiveModel
 from etna.models.prophet import ProphetModel
+from etna.models.sarimax import SARIMAXModel
 from etna.models.seasonal_ma import SeasonalMovingAverageModel