Source code for botorch.acquisition.multi_objective.utils
#!/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.
r"""
Utilities for multi-objective acquisition functions.
"""
from __future__ import annotations
import math
import warnings
from typing import Callable, List, Optional
import torch
from botorch import settings
from botorch.acquisition import monte_carlo # noqa F401
from botorch.acquisition.multi_objective.objective import (
IdentityMCMultiOutputObjective,
MCMultiOutputObjective,
)
from botorch.exceptions.errors import UnsupportedError
from botorch.exceptions.warnings import BotorchWarning, SamplingWarning
from botorch.models.model import Model
from botorch.sampling.samplers import IIDNormalSampler, SobolQMCNormalSampler
from botorch.utils.multi_objective.pareto import is_non_dominated
from botorch.utils.transforms import normalize_indices
from torch import Tensor
from torch.quasirandom import SobolEngine
[docs]def get_default_partitioning_alpha(num_objectives: int) -> float:
r"""Determines an approximation level based on the number of objectives.
If `alpha` is 0, FastNondominatedPartitioning should be used. Otherwise,
an approximate NondominatedPartitioning should be used with approximation
level `alpha`.
Args:
num_objectives: the number of objectives.
Returns:
The approximation level `alpha`.
"""
if num_objectives <= 4:
return 0.0
elif num_objectives > 6:
warnings.warn("EHVI works best for less than 7 objectives.", BotorchWarning)
return 10 ** (-8 + num_objectives)
[docs]def prune_inferior_points_multi_objective(
model: Model,
X: Tensor,
ref_point: Tensor,
objective: Optional[MCMultiOutputObjective] = None,
constraints: Optional[List[Callable[[Tensor], Tensor]]] = None,
num_samples: int = 2048,
max_frac: float = 1.0,
marginalize_dim: Optional[int] = None,
) -> Tensor:
r"""Prune points from an input tensor that are unlikely to be pareto optimal.
Given a model, an objective, and an input tensor `X`, this function returns
the subset of points in `X` that have some probability of being pareto
optimal, better than the reference point, and feasible. This function uses
sampling to estimate the probabilities, the higher the number of points `n`
in `X` the higher the number of samples `num_samples` should be to obtain
accurate estimates.
Args:
model: A fitted model. Batched models are currently not supported.
X: An input tensor of shape `n x d`. Batched inputs are currently not
supported.
ref_point: The reference point.
objective: The objective under which to evaluate the posterior.
constraints: A list of callables, each mapping a Tensor of dimension
`sample_shape x batch-shape x q x m` to a Tensor of dimension
`sample_shape x batch-shape x q`, where negative values imply
feasibility.
num_samples: The number of samples used to compute empirical
probabilities of being the best point.
max_frac: The maximum fraction of points to retain. Must satisfy
`0 < max_frac <= 1`. Ensures that the number of elements in the
returned tensor does not exceed `ceil(max_frac * n)`.
marginalize_dim: A batch dimension that should be marginalized.
For example, this is useful when using a batched fully Bayesian
model.
Returns:
A `n' x d` with subset of points in `X`, where
n' = min(N_nz, ceil(max_frac * n))
with `N_nz` the number of points in `X` that have non-zero (empirical,
under `num_samples` samples) probability of being pareto optimal.
"""
if X.ndim > 2:
# TODO: support batched inputs (req. dealing with ragged tensors)
raise UnsupportedError(
"Batched inputs `X` are currently unsupported by "
"prune_inferior_points_multi_objective"
)
max_points = math.ceil(max_frac * X.size(-2))
if max_points < 1 or max_points > X.size(-2):
raise ValueError(f"max_frac must take values in (0, 1], is {max_frac}")
with torch.no_grad():
posterior = model.posterior(X=X)
if posterior.event_shape.numel() > SobolEngine.MAXDIM:
if settings.debug.on():
warnings.warn(
f"Sample dimension q*m={posterior.event_shape.numel()} exceeding Sobol "
f"max dimension ({SobolEngine.MAXDIM}). Using iid samples instead.",
SamplingWarning,
)
sampler = IIDNormalSampler(num_samples=num_samples)
else:
sampler = SobolQMCNormalSampler(num_samples=num_samples)
samples = sampler(posterior)
if objective is None:
objective = IdentityMCMultiOutputObjective()
obj_vals = objective(samples, X=X)
if obj_vals.ndim > 3:
if obj_vals.ndim == 4 and marginalize_dim is not None:
obj_vals = obj_vals.mean(dim=marginalize_dim)
else:
# TODO: support batched inputs (req. dealing with ragged tensors)
raise UnsupportedError(
"Models with multiple batch dims are currently unsupported by"
" prune_inferior_points_multi_objective."
)
if constraints is not None:
infeas = torch.stack([c(samples) > 0 for c in constraints], dim=0).any(dim=0)
if infeas.ndim == 3 and marginalize_dim is not None:
# make sure marginalize_dim is not negative
if marginalize_dim < 0:
# add 1 to the normalize marginalize_dim since we have already
# removed the output dim
marginalize_dim = (
1 + normalize_indices([marginalize_dim], d=infeas.ndim)[0]
)
infeas = infeas.float().mean(dim=marginalize_dim).round().bool()
# set infeasible points to be the ref point
obj_vals[infeas] = ref_point
pareto_mask = is_non_dominated(obj_vals, deduplicate=False) & (
obj_vals > ref_point
).all(dim=-1)
probs = pareto_mask.to(dtype=X.dtype).mean(dim=0)
idcs = probs.nonzero().view(-1)
if idcs.shape[0] > max_points:
counts, order_idcs = torch.sort(probs, descending=True)
idcs = order_idcs[:max_points]
effective_n_w = obj_vals.shape[-2] // X.shape[-2]
idcs = (idcs / effective_n_w).long().unique()
return X[idcs]
