class_docstr: A class to represent ordering an amount of a financial instrument.
  
  Responsibilities of the Order:
      1. Confirming its own validity.
      2. Tracking its trades and reporting it back to the broker.
      3. Managing movement of quantities from order to order.
      4. Generating the next order in its path given that there is a
         'OrderSpec' for how to make the next order.
      5. Managing its own state changes when it can.
  
  Parameters
  ----------
   side : `TradeSide`
      The side of the order.
  exchange_pair : `ExchangePair`
      The exchange pair to perform the order for.
  price : float
      The price of the order.
  trade_type : `TradeType`
      The type of trade being made.
  exchange_pair : `ExchangePair`
      The exchange pair that the order is made for.
  quantity : `Quantity`
      The quantity of the order.
  portfolio : `Portfolio`
      The portfolio being used in the order.
  criteria : `Callable[[Order, Exchange], bool]`, optional
      The criteria under which the order will be considered executable.
  path_id : str, optional
      The path order id.
  start : int, optional
      The start time of the order.
  end : int, optional
      The end time of the order.
  
  Raises
  ------
  InvalidOrderQuantity
      Raised if the given quantity has a size of 0.
docstr: The quote instrument of the pair being traded.
code: def quote_instrument(self) -> "Instrument":
      """The quote instrument of the pair being traded."""
      return self.exchange_pair.pair.quote
  
imports: import uuid
  from enum import Enum
  from typing import Callable
  from decimal import Decimal
  from tensortrade.core import TimedIdentifiable, Observable
  from tensortrade.core.exceptions import InvalidOrderQuantity
  from tensortrade.oms.instruments import Quantity, ExchangePair
  from tensortrade.oms.orders import Trade, TradeSide, TradeType
label_desc: Use image node_id 5 for calling the Order obj's underlying member method code with example usage: obj.quote_instrument() and returns: self