Migrate GS usages to new module location

Summary:
X-link: facebookresearch/aepsych#623

X-link: facebook/Ax#3311

In this diff, we migrated everything in the codebase to use `ax.generation_strategy.xxx` instead of the old `ax.modelbridge.xxx` for the following files.

```
best_model_selector.py
dispatch_utils.py
external_generation_node.py
generation_node_input_constructors.py
generation_node.py
generation_strategy.py
model_spec.py
transition_criterion.py
```

Meta: In the previous diff D68720587 we moved these files from `ax/modelbridge/` to the new `ax/generation_strategy` directory, but all applications are still using/importing from the `modelbridge` folder.

Reviewed By: saitcakmak

Differential Revision: D68645075

fbshipit-source-id: e724c73e3733a04d6c5e8fa31284473095451e75

kats/utils/time_series_parameter_tuning.py

@@ -39,10 +39,10 @@
 from ax.core.objective import Objective
 from ax.core.outcome_constraint import OutcomeConstraint
 from ax.core.trial import BaseTrial
+from ax.generation_strategy.dispatch_utils import choose_generation_strategy
 from ax.global_stopping.strategies.improvement import ImprovementGlobalStoppingStrategy
 from ax.modelbridge.base import Adapter
 from ax.modelbridge.discrete import DiscreteAdapter
-from ax.modelbridge.dispatch_utils import choose_generation_strategy
 from ax.modelbridge.registry import Generators
 from ax.runners.synthetic import SyntheticRunner
 from ax.service.scheduler import Scheduler, SchedulerOptions