Ease-of-use improvements to wrapper teacher

[EricMichaelSmith]

Patch description
In most cases, AbstractWrapperTeacher is used to edit the fields of the action returned by the teacher that it wraps around. Assuming that this inner teacher is a FixedDialogTeacher, however, editing the fields of this action is tricky because it must be done before the FixedDialogTeacher performs further processing on the action and registers it for metrics and evaluations, as part of FixedDialogTeacher.process_action(). This, this PR gives AbstractWrapperTeacher a concrete implementation of .act() and an abstract method ._edit_action(), in order to give subclasses of AbstractWrapperTeacher an easy way to edit the action at the appropriate point in the pipeline without worrying about side effects.

This change is back-compatible with all existing subclasses of AbstractWrapperTeacher, which are not required to use the new .act() method or to provide an implementation of ._edit_action(). However, two particular subclasses of AbstractWrapperTeacher have been upgraded to use ._edit_action() to demonstrate how this new method is used.

Testing steps
This change has test coverage through tests/nightly/gpu/test_style_gen.py and tests/test_wrapper.py.

test_new_tasks.py
As expected, an error is produced when running python tests/datatests/test_new_tasks.py on AbstractWrapperTeacher, for the reason given in #2642

[github-actions]

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python tests/datatests/test_new_tasks.py

[klshuster]

lgtm!

[dianaglzrico]

@EricMichaelSmith sorry, just saw this but this looks really nice! it will help to understand better for any future users, thanks for doing this!