Codegen only the attribute classes that are used

[olsl21]

Description of issue

Currently, the generator generates all the Attribute classes in _attributes.py.mako for all drivers even if some of them are not used. We should make the generator smarter so that only the Attribute classes that are used get generated.

Kindly refer to https://github.com/ni/nimi-python/pull/1738#discussion_r904141387 for more details.