Updated to TensorFlow 2.3.1

Switched to using TensorFlow 2.3.1 that fixes multiple
security vulnerabilities of TensorFlow 2.3.0. Also:

• Fixed a minor bug related to optimizer slot variables not being saved in TF checkpoints when tf.stopGradients is used.
• Switched to supporting three native platforms: linux, windows, and darwin, where the last one does not include GPU support.