Make HEBO sampler supports Define-by-Run manner, maximization, parallelization, and constant_liar

package/samplers/hebo/README.md

@@ -20,16 +20,54 @@ cd HEBO/HEBO
 pip install -e .
 ```
 
+## APIs
+
+- `HEBOSampler(search_space: dict[str, BaseDistribution] | None = None, *, seed: int | None = None, constant_liar: bool = False, independent_sampler: BaseSampler | None = None)`
+  - `search_space`: By specifying search_space, the sampling speed at each iteration becomes slightly quicker, but this argument is not necessary to run this sampler.
+
+    Example:
+
+    ```python
+    search_space = {
+        "x": optuna.distributions.FloatDistribution(-5, 5),
+        "y": optuna.distributions.FloatDistribution(-5, 5),
+    }
+    HEBOSampler(search_space=search_space)
+    ```
+
+  - `seed`: Seed for random number generator.
+
+  - `constant_liar`: If `True`, penalize running trials to avoid suggesting parameter configurations nearby. Default is `False`.
+
+    - Note: Abnormally terminated trials often leave behind a record with a state of `RUNNING` in the storage. Such "zombie" trial parameters will be avoided by the constant liar algorithm during subsequent sampling. When using an `optuna.storages.RDBStorage`, it is possible to enable the `heartbeat_interval` to change the records for abnormally terminated trials to `FAIL`. (This note is quoted from [TPESampler](https://github.com/optuna/optuna/blob/v4.1.0/optuna/samplers/_tpe/sampler.py#L215-L222).)
+    - Note: It is recommended to set this value to `True` during distributed optimization to avoid having multiple workers evaluating similar parameter configurations. In particular, if each objective function evaluation is costly and the durations of the running states are significant, and/or the number of workers is high. (This note is quoted from [TPESampler](https://github.com/optuna/optuna/blob/v4.1.0/optuna/samplers/_tpe/sampler.py#L224-L229).)
+    - Note: HEBO algorithm involves multi-objective optimization of multiple acquisition functions. While `constant_liar` is a simple way to get diverse params for parallel optimization, it may not be the best approach for HEBO.
+
+  - `independent_sampler`: A `optuna.samplers.BaseSampler` instance that is used for independent sampling. The parameters not contained in the relative search space are sampled by this sampler. If `None` is specified, `optuna.samplers.RandomSampler` is used as the default.
+
 ## Example
 
 ```python
-search_space = {
-    "x": FloatDistribution(-10, 10),
-    "y": IntDistribution(0, 10),
+import optuna
+import optunahub
+
 
-}
-sampler = HEBOSampler(search_space)
+def objective(trial: optuna.trial.Trial) -> float:
+    x = trial.suggest_float("x", -10, 10)
+    y = trial.suggest_int("y", -10, 10)
+    return x**2 + y**2
+
+
+module = optunahub.load_module("samplers/hebo")
+sampler = module.HEBOSampler(search_space={
+    "x": optuna.distributions.FloatDistribution(-10, 10),
+    "y": optuna.distributions.IntDistribution(-10, 10),
+})
+# sampler = module.HEBOSampler()  # Note: `search_space` is not required, and thus it works too.
 study = optuna.create_study(sampler=sampler)
+study.optimize(objective, n_trials=100)
+
+print(study.best_trial.params, study.best_trial.value)
 ```
 
 See [`example.py`](https://github.com/optuna/optunahub-registry/blob/main/package/samplers/hebo/example.py) for a full example.

package/samplers/hebo/sampler.py

@@ -1,13 +1,19 @@
 from __future__ import annotations
 
 from collections.abc import Sequence
+from typing import Any
+import warnings
 
 import numpy as np
+import optuna
 from optuna.distributions import BaseDistribution
 from optuna.distributions import CategoricalDistribution
 from optuna.distributions import FloatDistribution
 from optuna.distributions import IntDistribution
+from optuna.samplers import BaseSampler
+from optuna.search_space import IntersectionSearchSpace
 from optuna.study import Study
+from optuna.study._study_direction import StudyDirection
 from optuna.trial import FrozenTrial
 from optuna.trial import TrialState
 import optunahub
@@ -18,19 +24,136 @@
 
 
 class HEBOSampler(optunahub.samplers.SimpleBaseSampler):
-    def __init__(self, search_space: dict[str, BaseDistribution]) -> None:
-        super().__init__(search_space)
-        self._hebo = HEBO(self._convert_to_hebo_design_space(search_space))
+    """A sampler using `HEBO <https://github.com/huawei-noah/HEBO/tree/master/HEBO>__` as the backend.
 
-    def sample_relative(
+    For further information about HEBO algorithm, please refer to the following paper:
+    - `HEBO Pushing The Limits of Sample-Efficient Hyperparameter Optimisation <https://arxiv.org/abs/2012.03826>__`
+
+    Args:
+        search_space:
+            By specifying search_space, the sampling speed at each iteration becomes slightly quicker, but this argument is not necessary to run this sampler. Default is :obj:`None`.
+
+        seed:
+            A seed for ``HEBOSampler``. Default is :obj:`None`.
+
+        constant_liar:
+            If :obj:`True`, penalize running trials to avoid suggesting parameter configurations
+            nearby. Default is :obj:`False`.
+
+            .. note::
+                Abnormally terminated trials often leave behind a record with a state of
+                ``RUNNING`` in the storage.
+                Such "zombie" trial parameters will be avoided by the constant liar algorithm
+                during subsequent sampling.
+                When using an :class:`~optuna.storages.RDBStorage`, it is possible to enable the
+                ``heartbeat_interval`` to change the records for abnormally terminated trials to
+                ``FAIL``.
+                (This note is quoted from `TPESampler <https://github.com/optuna/optuna/blob/v4.1.0/optuna/samplers/_tpe/sampler.py#L215-L222>__`.)
+
+            .. note::
+                It is recommended to set this value to :obj:`True` during distributed
+                optimization to avoid having multiple workers evaluating similar parameter
+                configurations. In particular, if each objective function evaluation is costly
+                and the durations of the running states are significant, and/or the number of
+                workers is high.
+                (This note is quoted from `TPESampler <https://github.com/optuna/optuna/blob/v4.1.0/optuna/samplers/_tpe/sampler.py#L224-L229>__`.)
+
+            .. note::
+                HEBO algorithm involves multi-objective optimization of multiple acquisition functions.
+                While `constant_liar` is a simple way to get diverse params for parallel optimization,
+                it may not be the best approach for HEBO.
+
+        independent_sampler:
+            A :class:`~optuna.samplers.BaseSampler` instance that is used for independent
+            sampling. The parameters not contained in the relative search space are sampled
+            by this sampler. If :obj:`None` is specified, :class:`~optuna.samplers.RandomSampler`
+            is used as the default.
+    """  # NOQA
+
+    def __init__(
+        self,
+        search_space: dict[str, BaseDistribution] | None = None,
+        *,
+        seed: int | None = None,
+        constant_liar: bool = False,
+        independent_sampler: BaseSampler | None = None,
+    ) -> None:
+        super().__init__(search_space, seed)
+        if search_space is not None and not constant_liar:
+            self._hebo = HEBO(self._convert_to_hebo_design_space(search_space), scramble_seed=seed)
+        else:
+            self._hebo = None
+        self._intersection_search_space = IntersectionSearchSpace()
+        self._independent_sampler = independent_sampler or optuna.samplers.RandomSampler(seed=seed)
+        self._is_fallback_inevitable = False
+        self._constant_liar = constant_liar
+        self._rng = np.random.default_rng(seed)
+
+    def _sample_relative_define_and_run(
         self, study: Study, trial: FrozenTrial, search_space: dict[str, BaseDistribution]
     ) -> dict[str, float]:
-        params_pd = self._hebo.suggest()
+        return {
+            name: row.iloc[0]
+            for name, row in self._hebo.suggest().items()
+            if name in search_space.keys()
+        }
 
-        params = {}
-        for name in search_space.keys():
-            params[name] = params_pd[name].to_numpy()[0]
-        return params
+    def _sample_relative_stateless(
+        self, study: Study, trial: FrozenTrial, search_space: dict[str, BaseDistribution]
+    ) -> dict[str, float]:
+        if self._constant_liar:
+            target_states = [TrialState.COMPLETE, TrialState.RUNNING]
+        else:
+            target_states = [TrialState.COMPLETE]
+
+        use_cache = not self._constant_liar
+        trials = study._get_trials(deepcopy=False, states=target_states, use_cache=use_cache)
+        if len([t for t in trials if t.state == TrialState.COMPLETE]) < 1:
+            # note: The backend HEBO implementation uses Sobol sampling here.
+            # This sampler does not call `hebo.suggest()` here because
+            # Optuna needs to know search space by running the first trial in Define-by-Run.
+            self._is_fallback_inevitable = True
+            return {}
+        else:
+            self._is_fallback_inevitable = False
+
+        # Assume that the back-end HEBO implementation aims to minimize.
+        if study.direction == StudyDirection.MINIMIZE:
+            worst_value = max(t.value for t in trials if t.state == TrialState.COMPLETE)
+        else:
+            worst_value = min(t.value for t in trials if t.state == TrialState.COMPLETE)
+        sign = 1 if study.direction == StudyDirection.MINIMIZE else -1
+
+        seed = self._rng.randint((1 << 31) - 1)
+        hebo = HEBO(self._convert_to_hebo_design_space(search_space), scramble_seed=seed)
+        valid_trials = [
+            t.params for t in trials if all(name in trial.params for name in search_space)
+        ]
+        params = pd.DataFrame([t.params for t in valid_trials])
+        values = np.array(
+            [
+                sign * t.value if t.state == TrialState.COMPLETE else worst_value
+                for t in valid_trials
+            ]
+        )
+        hebo.observe(params, values)
+        return {
+            name: row.iloc[0]
+            for name, row in hebo.suggest().items()
+            if name in search_space.keys()
+        }
+
+    def sample_relative(
+        self, study: Study, trial: FrozenTrial, search_space: dict[str, BaseDistribution]
+    ) -> dict[str, float]:
+        if study._is_multi_objective():
+            raise ValueError(
+                f"{self.__class__.__name__} has not supported multi-objective optimization."
+            )
+        if self._hebo is None or self._constant_liar is True:
+            return self._sample_relative_stateless(study, trial, search_space)
+        else:
+            return self._sample_relative_define_and_run(study, trial, search_space)
 
     def after_trial(
         self,
@@ -39,7 +162,11 @@ def after_trial(
         state: TrialState,
         values: Sequence[float] | None,
     ) -> None:
-        self._hebo.observe(pd.DataFrame([trial.params]), np.asarray([values]))
+        if self._hebo is not None and values is not None:
+            # Assume that the back-end HEBO implementation aims to minimize.
+            if study.direction == StudyDirection.MAXIMIZE:
+                values = [-x for x in values]
+            self._hebo.observe(pd.DataFrame([trial.params]), np.asarray([values]))
 
     def _convert_to_hebo_design_space(
         self, search_space: dict[str, BaseDistribution]
@@ -103,3 +230,26 @@ def _convert_to_hebo_design_space(
             else:
                 raise NotImplementedError(f"Unsupported distribution: {distribution}")
         return DesignSpace().parse(design_space)
+
+    def infer_relative_search_space(
+        self, study: Study, trial: FrozenTrial
+    ) -> dict[str, BaseDistribution]:
+        return optuna.search_space.intersection_search_space(
+            study._get_trials(deepcopy=False, use_cache=True)
+        )
+
+    def sample_independent(
+        self,
+        study: Study,
+        trial: FrozenTrial,
+        param_name: str,
+        param_distribution: BaseDistribution,
+    ) -> Any:
+        if not self._is_fallback_inevitable:
+            warnings.warn(
+                "`HEBOSampler` falls back to `RandomSampler` due to dynamic search space."
+            )
+
+        return self._independent_sampler.sample_independent(
+            study, trial, param_name, param_distribution
+        )