Fix maml_toy example

[DubiousCactus]

Description

The MAML Toy example examples/maml_toy.py does not run due to a sigma parameter being sampled from a Normal distribution. That sigma is used to parameterise another Normal distribution, hence must be positive.

Contribution Checklist

If your contribution modifies code in the core library (not docs, tests, or examples), please fill the following checklist.

• [x] My contribution is listed in CHANGELOG.md with attribution.
• [ ] My contribution modifies code in the main library.
• [x] My modifications are tested.
• [ ] My modifications are documented.