Move generation_strategy to a dedicated folder

Summary:
Moving the following from `ax/modelbridge/` to the new `ax/generation_strategy` directory

```
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
```

Reviewed By: saitcakmak

Differential Revision: D68720587

Author: ItsMrLin

ax/generation_strategy/__init__.py

@@ -0,0 +1,7 @@
+#!/usr/bin/env python3
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+# pyre-strict

ax/generation_strategy/best_model_selector.py

@@ -0,0 +1,131 @@
+#!/usr/bin/env python3
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+# pyre-strict
+
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from collections.abc import Callable
+from enum import Enum
+from functools import partial
+from typing import Any, Union
+
+import numpy as np
+import numpy.typing as npt
+from ax.exceptions.core import UserInputError
+from ax.generation_strategy.model_spec import ModelSpec
+from ax.utils.common.base import Base
+from pyre_extensions import none_throws
+
+# pyre-fixme[24]: Generic type `np.ndarray` expects 2 type parameters.
+ARRAYLIKE = Union[np.ndarray, list[float], list[np.ndarray]]
+
+
+class BestModelSelector(ABC, Base):
+    @abstractmethod
+    def best_model(self, model_specs: list[ModelSpec]) -> ModelSpec:
+        """Return the best ``ModelSpec`` based on some criteria.
+
+        NOTE: The returned ``ModelSpec`` may be a different object than
+        what was provided in the original list. It may be possible to
+        clone and modify the original ``ModelSpec`` to produce one that
+        performs better.
+        """
+
+
+class ReductionCriterion(Enum):
+    """An enum for callables that are used for aggregating diagnostics over metrics
+    and selecting the best diagnostic in ``SingleDiagnosticBestModelSelector``.
+
+    NOTE: This is used to ensure serializability of the callables.
+    """
+
+    # NOTE: Callables need to be wrapped in `partial` to be registered as members.
+    # pyre-fixme[35]: Target cannot be annotated.
+    MEAN: Callable[[ARRAYLIKE], npt.NDArray] = partial(np.mean)
+    # pyre-fixme[35]: Target cannot be annotated.
+    MIN: Callable[[ARRAYLIKE], npt.NDArray] = partial(np.min)
+    # pyre-fixme[35]: Target cannot be annotated.
+    MAX: Callable[[ARRAYLIKE], npt.NDArray] = partial(np.max)
+
+    def __call__(self, array_like: ARRAYLIKE) -> npt.NDArray:
+        return self.value(array_like)
+
+
+class SingleDiagnosticBestModelSelector(BestModelSelector):
+    """Choose the best model using a single cross-validation diagnostic.
+
+    The input is a list of ``ModelSpec``, each corresponding to one model.
+    The specified diagnostic is extracted from each of the models,
+    its values (each of which corresponds to a separate metric) are
+    aggregated with the aggregation function, the best one is determined
+    with the criterion, and the index of the best diagnostic result is returned.
+
+    Example:
+     ::
+        s = SingleDiagnosticBestModelSelector(
+            diagnostic='Fisher exact test p',
+            metric_aggregation=ReductionCriterion.MEAN,
+            criterion=ReductionCriterion.MIN,
+            model_cv_kwargs={"untransform": False},
+        )
+        best_model = s.best_model(model_specs=model_specs)
+
+    Args:
+        diagnostic: The name of the diagnostic to use, which should be
+            a key in ``CVDiagnostic``.
+        metric_aggregation: ``ReductionCriterion`` applied to the values of the
+            diagnostic for a single model to produce a single number.
+        criterion: ``ReductionCriterion`` used to determine which of the
+            (aggregated) diagnostics is the best.
+        model_cv_kwargs: Optional dictionary of kwargs to pass in while computing
+            the cross validation diagnostics.
+    """
+
+    def __init__(
+        self,
+        diagnostic: str,
+        metric_aggregation: ReductionCriterion,
+        criterion: ReductionCriterion,
+        model_cv_kwargs: dict[str, Any] | None = None,
+    ) -> None:
+        self.diagnostic = diagnostic
+        if not isinstance(metric_aggregation, ReductionCriterion) or not isinstance(
+            criterion, ReductionCriterion
+        ):
+            raise UserInputError(
+                "Both `metric_aggregation` and `criterion` must be "
+                f"`ReductionCriterion`. Got {metric_aggregation=}, {criterion=}."
+            )
+        if criterion == ReductionCriterion.MEAN:
+            raise UserInputError(
+                f"{criterion=} is not supported. Please use MIN or MAX."
+            )
+        self.metric_aggregation = metric_aggregation
+        self.criterion = criterion
+        self.model_cv_kwargs = model_cv_kwargs
+
+    def best_model(self, model_specs: list[ModelSpec]) -> ModelSpec:
+        """Return the best ``ModelSpec`` based on the specified diagnostic.
+
+        Args:
+            model_specs: List of ``ModelSpec`` to choose from.
+
+        Returns:
+            The best ``ModelSpec`` based on the specified diagnostic.
+        """
+        for model_spec in model_specs:
+            model_spec.cross_validate(model_cv_kwargs=self.model_cv_kwargs)
+        aggregated_diagnostic_values = [
+            self.metric_aggregation(
+                list(none_throws(model_spec.diagnostics)[self.diagnostic].values())
+            )
+            for model_spec in model_specs
+        ]
+        best_diagnostic = self.criterion(aggregated_diagnostic_values).item()
+        best_index = aggregated_diagnostic_values.index(best_diagnostic)
+        return model_specs[best_index]