tf.contrib.learn.monitors.BaseMonitor.step_begin(step)
Callback before training step begins.
You may use this callback to request evaluation of additional tensors in the graph.
Args:
step: int, the current value of the global step.
Returns:
List of Tensor objects or string tensor names to be run.
Raises:
ValueError: if we've already begun a step, or step < 0, or step > max_steps.