feat: Add public utility for per-sample gradient validation (#484)

README.md

@@ -131,6 +131,30 @@ of blogposts and talks:
 Check out the [FAQ](https://opacus.ai/docs/faq) page for answers to some of the
 most frequently asked questions about differential privacy and Opacus.
 
+## Verifying Per-Sample Gradients
+
+If you implement custom layers or grad samplers, you can use
+`get_per_sample_gradient_diagnostics` to verify that Opacus computes
+per-sample gradients correctly for your model. It compares Opacus's optimized
+computation against a reliable (but slow) micro-batch reference and returns a
+detailed per-parameter report.
+
+```python
+import torch
+from opacus.utils import get_per_sample_gradient_diagnostics
+
+model = MyCustomModel()
+x = torch.randn(8, 16)  # sample batch
+
+report = get_per_sample_gradient_diagnostics(x, model)
+if report["passed"]:
+    print("All per-sample gradients are correct!")
+else:
+    for name, p in report["reductions"]["mean"]["parameters"].items():
+        if not p["passed"]:
+            print(f"FAIL {name}: MSE={p['mse']:.2e}, L1={p['l1_loss']:.2e}")
+```
+
 ## Contributing
 
 See the

opacus/tests/per_sample_gradients_utils_test.py

@@ -15,13 +15,16 @@
 
 import unittest
 from typing import Callable
+from unittest.mock import patch
 
 import hypothesis.strategies as st
 import torch
 from hypothesis import given, settings
 from opacus.utils.per_sample_gradients_utils import (
     check_per_sample_gradients_are_correct,
+    compute_opacus_grad_sample,
     get_grad_sample_modes,
+    get_per_sample_gradient_diagnostics,
 )
 from torch import nn
 
@@ -152,3 +155,98 @@ def test_linear(
             return
 
         self.per_sample_grads_utils_test(x, linear, grad_sample_mode, N == 0)
+
+
+class DiagnosticsUtilsTest(unittest.TestCase):
+    """Tests for the public get_per_sample_gradient_diagnostics API."""
+
+    def test_diagnostics_output_structure(self):
+        """Verify the diagnostics report has the expected dictionary structure."""
+        model = nn.Linear(8, 4)
+        x = torch.randn(2, 8)
+
+        report = get_per_sample_gradient_diagnostics(x, model)
+
+        # Top-level keys
+        self.assertIn("passed", report)
+        self.assertIn("num_parameters", report)
+        self.assertIn("reductions", report)
+        self.assertIsInstance(report["passed"], bool)
+        self.assertIsInstance(report["num_parameters"], int)
+
+        # Reduction-level keys
+        for reduction in ["sum", "mean"]:
+            self.assertIn(reduction, report["reductions"])
+            red_report = report["reductions"][reduction]
+            self.assertIn("passed", red_report)
+            self.assertIn("parameters", red_report)
+
+            # Parameter-level keys
+            for param_name, param_report in red_report["parameters"].items():
+                self.assertIn("passed", param_report)
+                self.assertIn("shape_match", param_report)
+                self.assertIn("opacus_shape", param_report)
+                self.assertIn("microbatch_shape", param_report)
+                self.assertIn("opacus_l2_norm", param_report)
+                self.assertIn("microbatch_l2_norm", param_report)
+                self.assertIn("mse", param_report)
+                self.assertIn("l1_loss", param_report)
+
+    def test_diagnostics_linear_passes(self):
+        """Verify that a standard nn.Linear model passes diagnostics."""
+        model = nn.Linear(10, 5, bias=True)
+        x = torch.randn(4, 10)
+
+        report = get_per_sample_gradient_diagnostics(x, model)
+
+        self.assertTrue(report["passed"])
+        self.assertEqual(report["num_parameters"], 2)  # weight + bias
+        for reduction in ["sum", "mean"]:
+            red_report = report["reductions"][reduction]
+            self.assertTrue(red_report["passed"])
+            for param_name, param_report in red_report["parameters"].items():
+                self.assertTrue(param_report["passed"])
+                self.assertTrue(param_report["shape_match"])
+
+    def test_layer_norm(self):
+        """Verify diagnostics pass for nn.LayerNorm."""
+        W = 8
+        model = nn.LayerNorm(W, elementwise_affine=True)
+        x = torch.randn(4, 6, W)
+
+        report = get_per_sample_gradient_diagnostics(x, model)
+        self.assertTrue(report["passed"])
+
+    def test_public_import_path(self):
+        """Verify the public import from opacus.utils works."""
+        from opacus.utils import (
+            get_per_sample_gradient_diagnostics as diag_fn,
+        )
+
+        self.assertTrue(callable(diag_fn))
+
+    def test_diagnostics_reports_mismatch(self):
+        """Verify the diagnostics report flags failure when per-sample gradients
+        do not match the micro-batch reference."""
+        model = nn.Linear(10, 5, bias=True)
+        x = torch.randn(4, 10)
+
+        def perturbed_compute_opacus_grad_sample(*args, **kwargs):
+            result = compute_opacus_grad_sample(*args, **kwargs)
+            return {name: g + 1.0 for name, g in result.items()}
+
+        with patch(
+            "opacus.utils.per_sample_gradients_utils.compute_opacus_grad_sample",
+            side_effect=perturbed_compute_opacus_grad_sample,
+        ):
+            report = get_per_sample_gradient_diagnostics(x, model)
+
+        self.assertFalse(report["passed"])
+        for reduction in ["sum", "mean"]:
+            red_report = report["reductions"][reduction]
+            self.assertFalse(red_report["passed"])
+            for param_report in red_report["parameters"].values():
+                self.assertFalse(param_report["passed"])
+                self.assertTrue(param_report["shape_match"])
+                self.assertGreater(param_report["mse"], 0.0)
+                self.assertGreater(param_report["l1_loss"], 0.0)

opacus/utils/__init__.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from .per_sample_gradients_utils import get_per_sample_gradient_diagnostics
+
+
+__all__ = [
+    "get_per_sample_gradient_diagnostics",
+]

opacus/utils/per_sample_gradients_utils.py

@@ -14,7 +14,7 @@
 # limitations under the License.
 
 import io
-from typing import Callable, Dict, Iterable, List, Union
+from typing import Any, Callable, Dict, Iterable, List, Union
 
 import numpy as np
 import torch
@@ -276,6 +276,187 @@ def check_per_sample_gradients_are_correct(
     return True
 
 
+def get_per_sample_gradient_diagnostics(
+    x: Union[torch.Tensor, PackedSequence],
+    module: nn.Module,
+    *,
+    batch_first: bool = True,
+    atol: float = 10e-6,
+    rtol: float = 10e-5,
+    grad_sample_mode: str = "hooks",
+) -> Dict[str, Any]:
+    """
+    Computes detailed diagnostics for per-sample gradient correctness.
+
+    This utility helps users verify that Opacus computes per-sample gradients
+    correctly for their specific model. It compares the result of the slow but
+    reliable micro-batch method (computing gradients one sample at a time) with
+    Opacus's optimized method, and returns a detailed report for each trainable
+    parameter.
+
+    This is particularly useful when:
+    - Implementing custom ``grad_samplers`` for new layer types.
+    - Using ``functorch``-based gradient computation with complex architectures.
+    - Validating that a model's per-sample gradients are correct before
+      deploying it for privacy-sensitive training.
+
+    Args:
+        x: Sample input batch to run through the model.
+        module: The ``nn.Module`` to check. Should be the raw model
+            (not wrapped with ``GradSampleModule``).
+        batch_first: Whether batch size is the first dimension (as opposed
+            to the second). Defaults to True.
+        atol: The absolute tolerance parameter for ``torch.allclose``.
+            Defaults to 10e-6.
+        rtol: The relative tolerance parameter for ``torch.allclose``.
+            Defaults to 10e-5.
+        grad_sample_mode: The Opacus grad sample mode to use.
+            One of ``"hooks"``, ``"functorch"``, or ``"ew"``.
+            Defaults to ``"hooks"``.
+
+    Returns:
+        A dictionary with the following structure::
+
+            {
+                "passed": bool,  # True if all parameters pass for all reductions
+                "num_parameters": int,
+                "reductions": {
+                    "mean": {
+                        "passed": bool,
+                        "parameters": {
+                            "<param_name>": {
+                                "passed": bool,
+                                "shape_match": bool,
+                                "opacus_shape": tuple,
+                                "microbatch_shape": tuple,
+                                "opacus_l2_norm": float,
+                                "microbatch_l2_norm": float,
+                                "mse": float,  # Mean Squared Error
+                                "l1_loss": float,  # Mean Absolute Error
+                            },
+                            ...
+                        }
+                    },
+                    "sum": { ... }  # same structure as "mean"
+                }
+            }
+
+    Raises:
+        RuntimeError: If ``grad_sample_mode="ew"`` and ``batch_first`` is False
+            or the torch version is incompatible.
+        RuntimeError: If the input batch ``x`` is empty.
+
+    Example:
+        >>> import torch
+        >>> import torch.nn as nn
+        >>> from opacus.utils.per_sample_gradients_utils import (
+        ...     get_per_sample_gradient_diagnostics,
+        ... )
+        >>> model = nn.Linear(10, 5)
+        >>> x = torch.randn(4, 10)
+        >>> report = get_per_sample_gradient_diagnostics(x, model)
+        >>> report["passed"]
+        True
+    """
+    reductions = ["sum", "mean"]
+    if grad_sample_mode == "ew":
+        if not batch_first:
+            raise RuntimeError("Batch should be first dimension.")
+        if not check_torch_version_for_ew_sample():
+            raise RuntimeError(f"Unsupported torch version: {torch.__version__}.")
+
+    all_passed = True
+    reduction_results: Dict[str, Any] = {}
+
+    for loss_reduction in reductions:
+        reduction_report = _get_diagnostics_for_reduction(
+            x,
+            module,
+            batch_first=batch_first,
+            loss_reduction=loss_reduction,
+            atol=atol,
+            rtol=rtol,
+            grad_sample_mode=grad_sample_mode,
+        )
+        if not reduction_report["passed"]:
+            all_passed = False
+        reduction_results[loss_reduction] = reduction_report
+
+    return {
+        "passed": all_passed,
+        "num_parameters": len(next(iter(reduction_results.values()))["parameters"]),
+        "reductions": reduction_results,
+    }
+
+
+def _get_diagnostics_for_reduction(
+    x: Union[torch.Tensor, PackedSequence],
+    module: nn.Module,
+    batch_first: bool = True,
+    loss_reduction: str = "mean",
+    atol: float = 10e-6,
+    rtol: float = 10e-5,
+    grad_sample_mode: str = "hooks",
+) -> Dict[str, Any]:
+    """
+    Internal helper: computes per-parameter diagnostics for a single
+    loss reduction mode (``"mean"`` or ``"sum"``).
+    """
+    (
+        microbatch_grad_samples,
+        opacus_grad_samples,
+    ) = compute_grad_samples_microbatch_and_opacus(
+        x,
+        module,
+        batch_first=batch_first,
+        loss_reduction=loss_reduction,
+        grad_sample_mode=grad_sample_mode,
+    )
+
+    all_passed = True
+    parameters: Dict[str, Dict[str, Any]] = {}
+
+    for name, opacus_grad_sample in opacus_grad_samples.items():
+        microbatch_grad_sample = microbatch_grad_samples[name]
+
+        shape_match = opacus_grad_sample.shape == microbatch_grad_sample.shape
+
+        if shape_match:
+            values_close = bool(
+                torch.allclose(microbatch_grad_sample, opacus_grad_sample, atol, rtol)
+            )
+            mse = float(
+                torch.nn.functional.mse_loss(opacus_grad_sample, microbatch_grad_sample)
+            )
+            l1_loss = float(
+                torch.nn.functional.l1_loss(opacus_grad_sample, microbatch_grad_sample)
+            )
+        else:
+            values_close = False
+            mse = float("inf")
+            l1_loss = float("inf")
+
+        param_passed = shape_match and values_close
+        if not param_passed:
+            all_passed = False
+
+        parameters[name] = {
+            "passed": param_passed,
+            "shape_match": shape_match,
+            "opacus_shape": tuple(opacus_grad_sample.shape),
+            "microbatch_shape": tuple(microbatch_grad_sample.shape),
+            "opacus_l2_norm": float(opacus_grad_sample.norm(2)),
+            "microbatch_l2_norm": float(microbatch_grad_sample.norm(2)),
+            "mse": mse,
+            "l1_loss": l1_loss,
+        }
+
+    return {
+        "passed": all_passed,
+        "parameters": parameters,
+    }
+
+
 def compute_microbatch_grad_sample_tensor_or_seq(
     x: Union[torch.Tensor, PackedSequence],
     module: nn.Module,