Remove multi-surrogate support in MBM

Summary:
MBM with multiple surrogates was never supported e2e and is being deprecated. This diff:
- Adds a new `surrogate_spec: Surrogate` input to `BoTorchModel`, to replace `surrogate_specs: dict[str, Surrogate]` input.
- Raises a `DeprecationWarning` (as an exception) if `surrogate_specs` is passed in with multiple elements.
- Cleans up the code that was necessary to support multiple surrogates.

Differential Revision: D64875988

ax/modelbridge/tests/test_registry.py

@@ -30,7 +30,6 @@
 from ax.models.torch.botorch_modular.model import BoTorchModel, SurrogateSpec
 from ax.models.torch.botorch_modular.surrogate import Surrogate
 from ax.models.torch.botorch_moo import MultiObjectiveBotorchModel
-from ax.utils.common.constants import Keys
 from ax.utils.common.kwargs import get_function_argument_names
 from ax.utils.common.testutils import TestCase
 from ax.utils.testing.core_stubs import (
@@ -74,11 +73,9 @@ def test_botorch_modular(self) -> None:
         self.assertEqual(gpei.model.botorch_acqf_class, qExpectedImprovement)
         self.assertEqual(gpei.model.acquisition_class, Acquisition)
         self.assertEqual(gpei.model.acquisition_options, {"best_f": 0.0})
-        self.assertIsInstance(gpei.model.surrogates[Keys.AUTOSET_SURROGATE], Surrogate)
+        self.assertIsInstance(gpei.model.surrogate, Surrogate)
         # SingleTaskGP should be picked.
-        self.assertIsInstance(
-            gpei.model.surrogates[Keys.AUTOSET_SURROGATE].model, SingleTaskGP
-        )
+        self.assertIsInstance(gpei.model.surrogate.model, SingleTaskGP)
 
         gr = gpei.gen(n=1)
         self.assertIsNotNone(gr.best_arm_predictions)
@@ -96,14 +93,10 @@ def test_SAASBO(self) -> None:
         self.assertIsInstance(saasbo, TorchModelBridge)
         self.assertEqual(saasbo._model_key, "SAASBO")
         self.assertIsInstance(saasbo.model, BoTorchModel)
-        surrogate_specs = saasbo.model.surrogate_specs
+        surrogate_spec = saasbo.model.surrogate_spec
         self.assertEqual(
-            surrogate_specs,
-            {
-                "SAASBO_Surrogate": SurrogateSpec(
-                    botorch_model_class=SaasFullyBayesianSingleTaskGP
-                )
-            },
+            surrogate_spec,
+            SurrogateSpec(botorch_model_class=SaasFullyBayesianSingleTaskGP),
         )
         self.assertEqual(
             saasbo.model.surrogate.botorch_model_class, SaasFullyBayesianSingleTaskGP

ax/models/torch/botorch.py

@@ -265,6 +265,7 @@ def __init__(
             "instead. If you run into a use case that is not supported by MBM, "
             "please raise this with an issue at https://github.com/facebook/Ax",
             DeprecationWarning,
+            stacklevel=2,
         )
         self.model_constructor = model_constructor
         self.model_predictor = model_predictor

ax/models/torch/botorch_modular/acquisition.py

@@ -336,8 +336,7 @@ def optimize(
             )
             return candidates, acqf_values, arm_weights
 
-        # 2. Handle search spaces with discrete features.
-        # 2a. Handle the fully discrete search space.
+        # 2. Handle fully discrete search spaces.
         if optimizer in (
             "optimize_acqf_discrete",
             "optimize_acqf_discrete_local_search",
@@ -384,7 +383,7 @@ def optimize(
                 )
             return candidates, acqf_values, arm_weights
 
-        # 2b. Handle mixed search spaces that have discrete and continuous features.
+        # 3. Handle mixed search spaces that have discrete and continuous features.
         # Only sequential optimization is supported for `optimize_acqf_mixed`.
         candidates, acqf_values = optimize_acqf_mixed(
             acq_function=self.acqf,