6442 device specific heads for dynunet

[wyli]

Fixes #6442

Description

make self.heads safe for multi devices

Types of changes

• 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.