The primary changes are:
1. Use the >>> format for code samples. But I couldn't change the code in mixed_precision.py to use the >>> format, since an error is thrown if both the Keras mixed precision API and the graph rewrite API are used in the same process.
2. In the enable_mixed_precision_graph_rewrite docstring, compare the graph rewrite API with the Keras mixed precision API.
3. In the error message that occurs if you try to use both the graph rewrite and the Keras MP API, recommend the Keras API instead of the graph rewrite.
4. Add new sections to Policy docstring: "How a layer users its policy's variable dtype" and "How to write a layer that supports mixed precision and float64". The latter section might be more appropriate for the mixed precision guide, but the guide is already very long and its hard to fit in.
PiperOrigin-RevId: 298419101
Change-Id: I1936e1bcce401d02d4fdeb36b9866eb9cd40c421
