Fix unfold2d and add test (#443)

Summary:
Fixes #442.
TL;DR: F.pad has an unintuitive syntax (we need to indicated padding from last dimensions to first dimensions).
This PR fixes that in unfold2d and adds tests with non-symmetric pad/dilation/kernel_size/stride.

Pull Request resolved: #443

Reviewed By: karthikprasad

Differential Revision: D37451293

Pulled By: alexandresablayrolles

fbshipit-source-id: 0587325f4dcaa4d1dcd3068d9046825964b9d49f

opacus/tests/grad_samples/conv2d_test.py

@@ -19,6 +19,8 @@
 import torch
 import torch.nn as nn
 from hypothesis import given, settings
+from opacus.utils.tensor_utils import unfold2d
+from torch.testing import assert_allclose
 
 from .common import GradSampleHooks_test, expander, shrinker
 
@@ -68,3 +70,52 @@ def test_conv2d(
             groups=groups,
         )
         self.run_test(x, conv, batch_first=True, atol=10e-5, rtol=10e-4)
+
+    @given(
+        B=st.integers(1, 4),
+        C=st.sampled_from([1, 3, 32]),
+        H=st.integers(11, 17),
+        W=st.integers(11, 17),
+        k_w=st.integers(2, 3),
+        k_h=st.integers(2, 3),
+        stride_w=st.integers(1, 2),
+        stride_h=st.integers(1, 2),
+        pad_h=st.sampled_from([0, 2]),
+        pad_w=st.sampled_from([0, 2]),
+        dilation_w=st.integers(1, 3),
+        dilation_h=st.integers(1, 3),
+    )
+    @settings(deadline=10000)
+    def test_unfold2d(
+        self,
+        B: int,
+        C: int,
+        H: int,
+        W: int,
+        k_w: int,
+        k_h: int,
+        pad_w: int,
+        pad_h: int,
+        stride_w: int,
+        stride_h: int,
+        dilation_w: int,
+        dilation_h: int,
+    ):
+        X = torch.randn(B, C, H, W)
+        X_unfold_torch = torch.nn.functional.unfold(
+            X,
+            kernel_size=(k_h, k_w),
+            padding=(pad_h, pad_w),
+            stride=(stride_w, stride_h),
+            dilation=(dilation_w, dilation_h),
+        )
+
+        X_unfold_opacus = unfold2d(
+            X,
+            kernel_size=(k_h, k_w),
+            padding=(pad_h, pad_w),
+            stride=(stride_w, stride_h),
+            dilation=(dilation_w, dilation_h),
+        )
+
+        assert_allclose(X_unfold_torch, X_unfold_opacus, atol=0, rtol=0)

opacus/utils/tensor_utils.py

@@ -130,7 +130,8 @@ def unfold2d(
     W_effective = (
         W + 2 * padding[1] - (kernel_size[1] + (kernel_size[1] - 1) * (dilation[1] - 1))
     ) // stride[1] + 1
-    input = F.pad(input, (padding[0], padding[0], padding[1], padding[1]))
+    # F.pad's first argument is the padding of the *last* dimension
+    input = F.pad(input, (padding[1], padding[1], padding[0], padding[0]))
     *shape_pad, H_pad, W_pad = input.shape
     strides = list(input.stride())
     strides = strides[:-2] + [