Bump optax from 0.1.4 to 0.1.5 (#219)

Bumps [optax](https://github.com/deepmind/optax) from 0.1.4 to 0.1.5.
- [Release notes](https://github.com/deepmind/optax/releases)
- [Commits](google-deepmind/optax@v0.1.4...v0.1.5)

---
updated-dependencies:
- dependency-name: optax
  dependency-type: indirect
  update-type: version-update:semver-patch
...

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