Summary:
Moves the base classes into their own file to avoid circular imports. Previosly, we couldn't import the base class and the log acqfs in the same file without introducing a circular import.

Also fixed typing of `eta`, as it cannot be `None`.

Differential Revision: D61721280

Summary:
Pull Request resolved: #2486

My first idea was to change this to use Log-EHVI, but this helper is used in the tutorial to compute the hypervolume, so we can't change the return value. Since this is not directly constructed by the user, raising a warning is not productive. This diff suppresses the warning.

Differential Revision: D61722790