6442 device specific heads for dynunet

[wyli]

Fixes #6442

Description

make self.heads safe for multi devices

Types of changes

• [x] Non-breaking change (fix or new feature that would not break existing functionality).
• [ ] Breaking change (fix or new feature that would cause existing functionality to change).
• [ ] New tests added to cover the changes.
• [ ] Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• [ ] Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• [ ] In-line docstrings updated.
• [ ] Documentation updated, tested make html command in the docs/ folder.

[wyli]

/build

[ericspod]

This does produce separate heads for each device/thread, however access to the dictionary needs to be synchronized somehow for thread-safety I think. I mentioned this just now too in #6442.