fix llm single column (#1247)

src/evidently/features/generated_features.py

@@ -71,7 +71,7 @@ def as_column(self, subcolumn: Optional[str] = None) -> "ColumnName":
                 f"Please specify subcolumn for {self.__class__.__name__} feature, possible values: "
                 + ", ".join(self._extract_subcolumn_name(c.name) for c in columns)
             )
-        if len(columns) == 1 and subcolumn is not None:
+        if len(columns) == 1 and subcolumn is not None and self._create_column_name(subcolumn) != columns[0].name:
             raise ValueError(f"{self.__class__.__name__} feature do not have subcolumns")
         try:
             fullname = self._create_column_name(subcolumn)