#!/usr/bin/env python3
# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
r"""
Sampler modules to be used with MC-evaluated acquisition functions.
"""
from abc import ABC, abstractmethod
from typing import Optional
import torch
from torch import Tensor
from torch.nn import Module
from torch.quasirandom import SobolEngine
from ..exceptions import UnsupportedError
from ..posteriors import Posterior
from ..utils.sampling import draw_sobol_normal_samples, manual_seed
[docs]class MCSampler(Module, ABC):
r"""Abstract base class for Samplers.
Subclasses must implement the `_construct_base_samples` method.
Attributes:
sample_shape: The shape of each sample.
resample: If `True`, re-draw samples in each `forward` evaluation -
this results in stochastic acquisition functions (and thus should
not be used with deterministic optimization algorithms).
collapse_batch_dims: If True, collapse the t-batch dimensions of the
produced samples to size 1. This is useful for preventing sampling
variance across t-batches.
Example:
This method is usually not called directly, but via the sampler's
`__call__` method:
>>> posterior = model.posterior(test_X)
>>> samples = sampler(posterior)
"""
[docs] def forward(self, posterior: Posterior) -> Tensor:
r"""Draws MC samples from the posterior.
Args:
posterior: The Posterior to sample from.
Returns:
The samples drawn from the posterior.
"""
base_sample_shape = self._get_base_sample_shape(posterior=posterior)
self._construct_base_samples(posterior=posterior, shape=base_sample_shape)
samples = posterior.rsample(
sample_shape=self.sample_shape, base_samples=self.base_samples
)
return samples
def _get_base_sample_shape(self, posterior: Posterior) -> torch.Size:
r"""Get the shape of the base samples.
Args:
posterior: The Posterior to sample from.
Returns:
The shape of the base samples expected by the posterior. If
`collapse_batch_dims=True`, the t-batch dimensions of the base
samples are collapsed to size 1. This is useful to prevent sampling
variance across t-batches.
"""
event_shape = posterior.event_shape
if self.collapse_batch_dims:
event_shape = torch.Size([1 for _ in event_shape[:-2]]) + event_shape[-2:]
return self.sample_shape + event_shape
@property
def sample_shape(self) -> torch.Size:
r"""The shape of a single sample"""
return self._sample_shape
@abstractmethod
def _construct_base_samples(self, posterior: Posterior, shape: torch.Size) -> None:
r"""Generate base samples (if necessary).
This function will generate a new set of base samples and register the
`base_samples` buffer if one of the following is true:
- `resample=True`
- the MCSampler has no `base_samples` attribute.
- `shape` is different than `self.base_samples.shape`.
- device and/or dtype of posterior are different than those of
`self.base_samples`.
Args:
posterior: The Posterior for which to generate base samples.
shape: The shape of the base samples to construct.
"""
pass # pragma: no cover
[docs]class IIDNormalSampler(MCSampler):
r"""Sampler for MC base samples using iid N(0,1) samples.
Example:
>>> sampler = IIDNormalSampler(1000, seed=1234)
>>> posterior = model.posterior(test_X)
>>> samples = sampler(posterior)
"""
def __init__(
self,
num_samples: int,
resample: bool = False,
seed: Optional[int] = None,
collapse_batch_dims: bool = True,
) -> None:
r"""Sampler for MC base samples using iid `N(0,1)` samples.
Args:
num_samples: The number of samples to use.
resample: If `True`, re-draw samples in each `forward` evaluation -
this results in stochastic acquisition functions (and thus should
not be used with deterministic optimization algorithms).
seed: The seed for the RNG. If omitted, use a random seed.
collapse_batch_dims: If True, collapse the t-batch dimensions to
size 1. This is useful for preventing sampling variance across
t-batches.
"""
super().__init__()
self._sample_shape = torch.Size([num_samples])
self.collapse_batch_dims = collapse_batch_dims
self.resample = resample
self.seed = seed if seed is not None else torch.randint(0, 1000000, (1,)).item()
def _construct_base_samples(self, posterior: Posterior, shape: torch.Size) -> None:
r"""Generate iid `N(0,1)` base samples (if necessary).
This function will generate a new set of base samples and set the
`base_samples` buffer if one of the following is true:
- `resample=True`
- the MCSampler has no `base_samples` attribute.
- `shape` is different than `self.base_samples.shape`.
- device and/or dtype of posterior ar different than those of
`self.base_samples`.
Args:
posterior: The Posterior for which to generate base samples.
shape: The shape of the base samples to construct.
"""
if (
self.resample
or not hasattr(self, "base_samples")
or self.base_samples.shape != shape
or self.base_samples.device != posterior.device
or self.base_samples.dtype != posterior.dtype
):
with manual_seed(seed=self.seed):
base_samples = torch.randn(
shape, device=posterior.device, dtype=posterior.dtype
)
self.seed += 1
self.register_buffer("base_samples", base_samples)
[docs]class SobolQMCNormalSampler(MCSampler):
r"""Sampler for quasi-MC base samples using Sobol sequences.
Example:
>>> sampler = SobolQMCNormalSampler(1000, seed=1234)
>>> posterior = model.posterior(test_X)
>>> samples = sampler(posterior)
"""
def __init__(
self,
num_samples: int,
resample: bool = False,
seed: Optional[int] = None,
collapse_batch_dims: bool = True,
) -> None:
r"""Sampler for quasi-MC base samples using Sobol sequences.
Args:
num_samples: The number of samples to use.
resample: If `True`, re-draw samples in each `forward` evaluation -
this results in stochastic acquisition functions (and thus should
not be used with deterministic optimization algorithms).
seed: The seed for the RNG. If omitted, use a random seed.
collapse_batch_dims: If True, collapse the t-batch dimensions to
size 1. This is useful for preventing sampling variance across
t-batches.
"""
super().__init__()
self._sample_shape = torch.Size([num_samples])
self.collapse_batch_dims = collapse_batch_dims
self.resample = resample
self.seed = seed if seed is not None else torch.randint(0, 1000000, (1,)).item()
def _construct_base_samples(self, posterior: Posterior, shape: torch.Size) -> None:
r"""Generate quasi-random Normal base samples (if necessary).
This function will generate a new set of base samples and set the
`base_samples` buffer if one of the following is true:
- `resample=True`
- the MCSampler has no `base_samples` attribute.
- `self.sample_shape` is different than `self.base_samples.shape`.
- device and/or dtype of posterior ar different than those of
`self.base_samples`.
Args:
posterior: The Posterior for which to generate base samples.
shape: The shape of the base samples to construct.
"""
if (
self.resample
or not hasattr(self, "base_samples")
or self.base_samples.shape != shape
or self.base_samples.device != posterior.device
or self.base_samples.dtype != posterior.dtype
):
output_dim = shape[-2:].numel()
if output_dim > SobolEngine.MAXDIM:
raise UnsupportedError(
"SobolQMCSampler only supports dimensions "
f"`q * o <= {SobolEngine.MAXDIM}`. Requested: {output_dim}"
)
base_samples = draw_sobol_normal_samples(
d=output_dim,
n=shape[:-2].numel(),
device=posterior.device,
dtype=posterior.dtype,
seed=self.seed,
)
self.seed += 1
base_samples = base_samples.view(shape)
self.register_buffer("base_samples", base_samples)
