class_docstr: Data source for session based data, the input data is organized in sessions,
  each session may have multiple rows. The first column is always the session id.
  Raw input rows are consolidated by session id and returned as one session
  per example
code: def __init__(self, id_col, **kwargs):
      self.id_col = id_col
      self.current_id = None
      self.current_session = []
      super().__init__(**kwargs)
  
file_path: pytext/pytext/data/sources/session.py
imports: from itertools import chain
  from typing import List
  from pytext.config.serialize import _get_class_type
  from pytext.data.sources.data_source import RootDataSource
label_desc: Use image node_id 1 to create a new SessionDataSource object from inherited base classes: RootDataSource with example: obj = SessionDataSource(id_col)