Remove fall-back to list (#2353)

Summary:
Pull Request resolved: #2353

Based on a recent discussion with the team, we decided to remove this special casing while we finalize GenerationNodes to make development a bit cleaner.

Reviewed By: lena-kashtelyan

Differential Revision: D55001898

fbshipit-source-id: 0c6b5a4b5ceb1caa619314347aa8a2bfd6124dea

Author: mgarrard

ax/modelbridge/generation_node.py

@@ -439,11 +439,9 @@ def should_transition_to_next_node(
                 raise NotImplementedError(
                     "Cannot currently select between multiple nodes to transition to."
                 )
-            elif len(next_nodes) == 1:
-                return True, next_nodes[0]
             else:
-                # Will transition to the next node in the list.
-                return True, None
+                return True, next_nodes[0]
+
         return False, None
 
     def generator_run_limit(self, supress_generation_errors: bool = True) -> int:

ax/modelbridge/tests/test_generation_strategy.py

@@ -1284,12 +1284,14 @@ def test_gs_with_nodes_and_blocking_criteria(self) -> None:
                     threshold=3,
                     block_gen_if_met=True,
                     block_transition_if_unmet=True,
+                    transition_to="GPEI_node",
                 ),
                 MinTrials(
                     threshold=2,
                     only_in_statuses=[TrialStatus.COMPLETED],
                     block_gen_if_met=False,
                     block_transition_if_unmet=True,
+                    transition_to="GPEI_node",
                 ),
             ],
         )

tutorials/external_generation_node.ipynb

@@ -223,9 +223,11 @@
         "            model_specs=[ModelSpec(Models.SOBOL)],\n",
         "            transition_criteria=[\n",
         "                MaxTrials(\n",
-        "                    # This specifies the maximum number of trials to generate from this node.\n",
+        "                    # This specifies the maximum number of trials to generate from this node, \n",
+        "                    # and the next node in the strategy.\n",
         "                    threshold=5,\n",
         "                    block_transition_if_unmet=True,\n",
+        "                    transition_to=\"RandomForest\"\n",
         "                )\n",
         "            ],\n",
         "        ),\n",