Update Tensorflow Prebuilt containers and saving method

[Matt-Viner]

Description

This repository has outdated versions of the GCP pre-built containers for training and serving Tensorflow models (2.6 is no longer being patched and will be removed from GCP in September 2024), so I have updated them to use version 2.12, this also requires a saving approach to change, as models saved with tf_model.save() method are not picked up by the pre-built prediction container. This has been updated to tf.saved_model.save() which should work ok.

How has this been tested?

I have made this change in an Org Repo that uses this template for training and serving tensorflow models, which ran successfully E2e. I don't personally have access to GCP or Vertex AI unfortunately!

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have successfully run the E2E tests, and have included the links to the pipeline runs below
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated any relevant documentation to reflect my changes
• I have assigned a reviewer and messaged them

Pipeline run links:

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