how to specify which gpu to use in training tensorflow code example

Example 1: gpu training tensorflow

import tensorflow as tf
print("Num GPUs Available: ", len(tf.config.experimental.list_physical_devices('GPU')))

Example 2: do i need do some set when i use GPU to train tensorflow model

from __future__ import absolute_import, division, print_function, unicode_literalsimport tensorflow as tfprint("Num GPUs Available: ", len(tf.config.experimental.list_physical_devices('GPU')))