'Initialize VariableDeviceChooser.
Args:
num_parameter_servers: number of parameter servers.
ps_device: string representing the parameter server device.
placement: string representing the placement of the variable either CPU:0
or GPU:0. When using parameter servers forced to CPU:0.'

def __init__(self, num_parameter_servers=0, ps_device='/job:ps', placement='CPU:0'):

    self._num_ps = num_parameter_servers
    self._ps_device = ps_device
    self._placement = (placement if (num_parameter_servers == 0) else 'CPU:0')
    self._next_task_id = 0
