class_docstr: Raw market data that is often used in backtesting (thus called BacktestData).
  
  Base class for all types of backtest data. Currently, each type of simulator has its corresponding backtest
  data type.
code: def __len__(self) -> int:
      raise NotImplementedError
  
file_path: qlib/qlib/rl/data/base.py
imports: from __future__ import annotations
  from abc import abstractmethod
  import pandas
label_desc: Use image node_id 2 for calling the BaseIntradayBacktestData obj's underlying member method code with example usage: obj.__len__() without return types