fix sensor.py bug

[FrankTianTT]

Hi, thank you for your brilliant job. I find a small bug in the class BoxSpaceSensor in sensor.py, as follows

if isinstance(lower_bound, float):
      self._lower_bound = np.full(shape, lower_bound, dtype=dtype)
    else:
      self._lower_bound = np.array(lower_bound)

but in the class BasePositionSensor in robot_sensors.py, the default parameters of constructor are

  def __init__(self,
               lower_bound: _FLOAT_OR_ARRAY = -100,
               upper_bound: _FLOAT_OR_ARRAY = 100,
               name: typing.Text = "BasePosition",
               dtype: typing.Type[typing.Any] = np.float64) -> None:
    """Constructs BasePositionSensor.
    Args:
      lower_bound: the lower bound of the base position of the robot.
      upper_bound: the upper bound of the base position of the robot.
      name: the name of the sensor
      dtype: data type of sensor value
    """
    super(BasePositionSensor, self).__init__(
        name=name,
        shape=(3,),  # x, y, z
        lower_bound=lower_bound,
        upper_bound=upper_bound,
        dtype=dtype)

So, you know, the result of isinstance(lower_bound, float) is False for BasePositionSensor, so BasePositionSensor._lower_bound become a float, but it should be a list, for example, [x, y, z].

So when I add sensor_wrappers.HistoricSensorWrapper(wrapped_sensor=robot_sensors.BasePositionSensor(), num_history=5) to the sensors, the observation returned by the step add 15, but env.observation_space.shape[0] only add 5!

I think it should help to correct class BoxSpaceSensor as follows

if isinstance(lower_bound, float) or isinstance(lower_bound, int):
      self._lower_bound = np.full(shape, lower_bound, dtype=dtype)
    else:
      self._lower_bound = np.array(lower_bound)

Thanks again for your outstanding work :)

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[FrankTianTT]

@googlebot I signed it!

------------------ 原始邮件 ------------------ 发件人: "googlebot"<notifications@github.com>; 发送时间: 2020年7月25日(星期六) 上午8:54 收件人: "google-research/motion_imitation"<motion_imitation@noreply.github.com>; 抄送: "田鸿龙"<franktian424@qq.com>; "Author"<author@noreply.github.com>; 主题: Re: [google-research/motion_imitation] fix sensor.py bug (#11)

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[erwincoumans]

Thanks for the PR!