Adding ordinal likelihood

docs/source/likelihoods.rst

@@ -94,6 +94,12 @@ reduce the variance when computing approximate GP objective functions.
 .. autoclass:: StudentTLikelihood
    :members:
 
+:hidden:`OrdinalLikelihood`
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: OrdinalLikelihood
+   :members:
+
 
 Multi-Dimensional Likelihoods
 -----------------------------

gpytorch/likelihoods/__init__.py

@@ -18,6 +18,7 @@
 from .multitask_gaussian_likelihood import _MultitaskGaussianLikelihoodBase, MultitaskGaussianLikelihood
 from .negative_binomial_likelihood import NegativeBinomialLikelihood
 from .noise_models import HeteroskedasticNoise
+from .ordinal_likelihood import OrdinalLikelihood
 from .poisson_likelihood import PoissonLikelihood
 from .softmax_likelihood import SoftmaxLikelihood
 from .student_t_likelihood import StudentTLikelihood
@@ -39,6 +40,7 @@
     "LikelihoodList",
     "MultitaskGaussianLikelihood",
     "NegativeBinomialLikelihood",
+    "OrdinalLikelihood",
     "PoissonLikelihood",
     "SoftmaxLikelihood",
     "StudentTLikelihood",

gpytorch/likelihoods/ordinal_likelihood.py

@@ -0,0 +1,108 @@
+from typing import Any
+
+import torch
+from torch import Tensor
+from torch.distributions import Categorical
+
+from ..constraints import Interval, Positive
+from ..priors import Prior
+from .likelihood import _OneDimensionalLikelihood
+
+
+def inv_probit(x, jitter=1e-3):
+    """
+    Inverse probit function (standard normal CDF) with jitter for numerical stability.
+
+    Args:
+        x: Input tensor
+        jitter: Small constant to ensure outputs are strictly between 0 and 1
+
+    Returns:
+        Probabilities between jitter and 1-jitter
+    """
+    return 0.5 * (1.0 + torch.erf(x / torch.sqrt(torch.tensor(2.0)))) * (1 - 2 * jitter) + jitter
+
+
+class OrdinalLikelihood(_OneDimensionalLikelihood):
+    r"""
+    An ordinal likelihood for regressing over ordinal data.
+
+    The data are integer values from :math:`0` to :math:`k`, and the user must specify :math:`(k-1)`
+    'bin edges' which define the points at which the labels switch. Let the bin
+    edges be :math:`[a_0, a_1, ... a_{k-1}]`, then the likelihood is
+
+    .. math::
+        p(Y=0|F) &= \Phi((a_0 - F) / \sigma)
+
+        p(Y=1|F) &= \Phi((a_1 - F) / \sigma) - \Phi((a_0 - F) / \sigma)
+
+        p(Y=2|F) &= \Phi((a_2 - F) / \sigma) - \Phi((a_1 - F) / \sigma)
+
+        ...
+
+        p(Y=K|F) &= 1 - \Phi((a_{k-1} - F) / \sigma)
+
+    where :math:`\Phi` is the cumulative density function of a Gaussian (the inverse probit
+    function) and :math:`\sigma` is a parameter to be learned.
+
+    From Chu et Ghahramani, Journal of Machine Learning Research, 2005
+    [https://www.jmlr.org/papers/volume6/chu05a/chu05a.pdf].
+
+    :param bin_edges: A tensor of shape :math:`(k-1)` containing the bin edges.
+    :param batch_shape: The batch shape of the learned sigma parameter (default: []).
+    :param sigma_prior: Prior for sigma parameter :math:`\sigma`.
+    :param sigma_constraint: Constraint for sigma parameter :math:`\sigma`.
+
+    :ivar torch.Tensor bin_edges: :math:`\{a_i\}_{i=0}^{k-1}` bin edges
+    :ivar torch.Tensor sigma: :math:`\sigma` parameter (scale)
+    """
+
+    def __init__(
+        self,
+        bin_edges: Tensor,
+        batch_shape: torch.Size = torch.Size([]),
+        sigma_prior: Prior | None = None,
+        sigma_constraint: Interval | None = None,
+    ) -> None:
+        super().__init__()
+
+        self.num_bins = len(bin_edges) + 1
+        self.register_parameter("bin_edges", torch.nn.Parameter(bin_edges, requires_grad=False))
+
+        if sigma_constraint is None:
+            sigma_constraint = Positive()
+
+        self.raw_sigma = torch.nn.Parameter(torch.ones(*batch_shape, 1))
+        if sigma_prior is not None:
+            self.register_prior("sigma_prior", sigma_prior, lambda m: m.sigma, lambda m, v: m._set_sigma(v))
+
+        self.register_constraint("raw_sigma", sigma_constraint)
+
+    @property
+    def sigma(self) -> Tensor:
+        return self.raw_sigma_constraint.transform(self.raw_sigma)
+
+    @sigma.setter
+    def sigma(self, value: Tensor) -> None:
+        self._set_sigma(value)
+
+    def _set_sigma(self, value: Tensor) -> None:
+        if not torch.is_tensor(value):
+            value = torch.as_tensor(value).to(self.raw_sigma)
+        self.initialize(raw_sigma=self.raw_sigma_constraint.inverse_transform(value))
+
+    def forward(self, function_samples: Tensor, *args: Any, data: dict[str, Tensor] = {}, **kwargs: Any) -> Categorical:
+        # Compute scaled bin edges
+        scaled_edges = self.bin_edges / self.sigma
+        scaled_edges_left = torch.cat([scaled_edges, torch.tensor([torch.inf], device=scaled_edges.device)], dim=-1)
+        scaled_edges_right = torch.cat([torch.tensor([-torch.inf], device=scaled_edges.device), scaled_edges])
+
+        # Calculate cumulative probabilities using standard normal CDF (probit function)
+        function_samples = function_samples.unsqueeze(-1)
+        scaled_edges_left = scaled_edges_left.reshape(1, -1)
+        scaled_edges_right = scaled_edges_right.reshape(1, -1)
+        probs = inv_probit(scaled_edges_left - function_samples / self.sigma) - inv_probit(
+            scaled_edges_right - function_samples / self.sigma
+        )
+
+        return Categorical(probs=probs)

test/likelihoods/test_ordinal_likelihood.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python3
+
+import unittest
+
+import torch
+from torch.distributions import Distribution
+
+from gpytorch.likelihoods import OrdinalLikelihood
+from gpytorch.test.base_likelihood_test_case import BaseLikelihoodTestCase
+
+
+class TestOrdinalLikelihood(BaseLikelihoodTestCase, unittest.TestCase):
+    seed = 0
+
+    def create_likelihood(self):
+        bin_edges = torch.tensor([-0.5, 0.5])
+        return OrdinalLikelihood(bin_edges)
+
+    def _create_targets(self, batch_shape=torch.Size([])):
+        return torch.distributions.Categorical(probs=torch.tensor([1 / 3, 1 / 3, 1 / 3])).sample(
+            torch.Size([*batch_shape, 5])
+        )
+
+    def _test_marginal(self, batch_shape):
+        likelihood = self.create_likelihood()
+        input = self._create_marginal_input(batch_shape)
+        output = likelihood(input)
+
+        self.assertTrue(isinstance(output, Distribution))
+        self.assertEqual(output.sample().shape[-len(batch_shape) - 1 :], torch.Size([*batch_shape, 5]))