In the tff.program library, there is usage of various `flatten` functions:

* `tf.nest.flatten`
* `tree.flatten`
* `structure_utils.flatten`, which is a wrapper around `tree.flatten_with_path`

This change does two things:

1. Consolidates usage of various `flatten` functions to the `tree` package, `tree` is used over `tf.nest` because it has more functionality.
2. Renames `structure_utils.flatten` to `structure_utils.flatten_with_name` to disambiguate this functionality.

PiperOrigin-RevId: 410910920