improve numerical stability in moo tests (facebook#679)

Summary:
Pull Request resolved: facebook#679

This should help with stochastic numerical issues

Reviewed By: Balandat

Differential Revision: D30530479

fbshipit-source-id: 918745994bea60552fcb4896eaa5a7e6d32ea144

ax/modelbridge/tests/test_multi_objective_torch_modelbridge.py

@@ -38,6 +38,7 @@
 from ax.utils.testing.core_stubs import (
     get_branin_data_multi_objective,
     get_branin_experiment_with_multi_objective,
+    TEST_SOBOL_SEED,
 )
 from botorch.utils.multi_objective.pareto import is_non_dominated
 
@@ -506,7 +507,14 @@ def test_infer_objective_thresholds(self, _, cuda=False):
             )
         )
         # test using MTGP
-        sobol_generator = get_sobol(search_space=exp.search_space)
+        sobol_generator = get_sobol(
+            search_space=exp.search_space,
+            seed=TEST_SOBOL_SEED,
+            # set initial position equal to the number of sobol arms generated
+            # so far. This means that new sobol arms will complement the previous
+            # arms in a space-filling fashion
+            init_position=len(exp.arms_by_name) - 1,
+        )
         sobol_run = sobol_generator.gen(n=5)
         trial = exp.new_batch_trial(optimize_for_power=True)
         trial.add_generator_run(sobol_run)

ax/utils/testing/core_stubs.py

@@ -84,6 +84,7 @@
 
 logger = get_logger(__name__)
 
+TEST_SOBOL_SEED = 1234
 
 ##############################
 # Experiments
@@ -381,8 +382,8 @@ def get_branin_experiment_with_multi_objective(
         exp.status_quo = Arm(parameters={"x1": 0.0, "x2": 0.0})
 
     if with_batch:
-        sobol_generator = get_sobol(search_space=exp.search_space)
-        sobol_run = sobol_generator.gen(n=15)
+        sobol_generator = get_sobol(search_space=exp.search_space, seed=TEST_SOBOL_SEED)
+        sobol_run = sobol_generator.gen(n=5)
         exp.new_batch_trial(optimize_for_power=with_status_quo).add_generator_run(
             sobol_run
         )