Customizable reduction strategy in model docs

[kretes]

Fills in the docs related to newly introduced distribute_reduction_method in the model for distributed training with custom train_step. Valid together with https://github.com/keras-team/keras/pull/16664

[github-actions[bot]]

Preview

Preview and run these notebook edits with Google Colab:

• site/en/guide/distributed_training.ipynb
• site/en/guide/tpu.ipynb
• site/en/tutorials/distribute/custom_training.ipynb
• site/en/tutorials/distribute/multi_worker_with_ctl.ipynb

Rendered notebook diffs available on ReviewNB.com.

Format and style

Use the TensorFlow docs notebook tools to format for consistent source diffs and lint for style:

$ python3 -m pip install -U --user git+https://github.com/tensorflow/docs

$ python3 -m tensorflow_docs.tools.nbfmt notebook.ipynb

$ python3 -m tensorflow_docs.tools.nblint --arg=repo:tensorflow/docs notebook.ipynb

If commits are added to the pull request, synchronize your local branch: git pull origin customizable-reduction-strategy-in-model

[kretes]

Given that we've implemented an "auto" strategy as default in https://github.com/keras-team/keras/pull/16664 - no updates to the docs are needed.