Update FeatureAblation to handle precision loss when baseline is more granular than input when cross tensor attribution is enabled (#1644)

Summary:

Noticed when flipping the flag, this test case failed:

https://www.internalfb.com/code/fbsource/[faf71541b1ec0fae639f82d487b81fb18ea3e523]/fbcode/pytorch/captum/tests/attr/test_dataloader_attr.py?lines=138%2C134

The ablated tensor was `tensor([0])` instead of `tensor([[0.1])` since the baseline was a float-type and the input tensors were int tensors.

https://www.internalfb.com/code/fbsource/[f2fcc926a6f3669602bac4d28c2d92e4197c96b9]/fbcode/pytorch/captum/captum/attr/_core/feature_ablation.py?lines=707-709

`ablated_input` is just a copy of the `input_tensor`, so during assignment, the ablated feature tensor incorrectly gets cast to an int tensor for this case.

Differential Revision: D81980219

Author: sarahtranfb

captum/attr/_core/feature_ablation.py

@@ -704,9 +704,11 @@ def _construct_ablated_input_across_tensors(
                 tensor_mask.append(mask)
 
                 assert baseline is not None, "baseline must be provided"
-                ablated_input[start_idx:end_idx] = input_tensor[start_idx:end_idx] * (
-                    1 - mask
+                ablated_feature = input_tensor[start_idx:end_idx] * (1 - mask).to(
+                    input_tensor.dtype
                 ) + (baseline * mask.to(input_tensor.dtype))
+                ablated_input = ablated_input.to(ablated_feature.dtype)
+                ablated_input[start_idx:end_idx] = ablated_feature
             current_masks.append(torch.stack(tensor_mask, dim=0))
             ablated_inputs.append(ablated_input)
 

tests/attr/test_feature_ablation.py

@@ -220,6 +220,42 @@ def test_multi_input_ablation_with_mask(self) -> None:
             perturbations_per_eval=(1, 2, 3),
         )
 
+    def test_multi_input_ablation_with_int_input_tensor_and_float_baseline(
+        self,
+    ) -> None:
+        def sum_forward(*inps: torch.Tensor) -> torch.Tensor:
+            flattened = [torch.flatten(inp, start_dim=1) for inp in inps]
+            return torch.cat(flattened, dim=1).sum(1)
+
+        ablation_algo = FeatureAblation(sum_forward)
+        inp1 = torch.tensor([[0, 1], [3, 4]])
+        inp2 = torch.tensor(
+            [
+                [[0.1, 0.2], [0.3, 0.2]],
+                [[0.4, 0.5], [0.3, 0.2]],
+            ]
+        )
+        inp3 = torch.tensor([[0], [1]])
+
+        expected = (
+            torch.tensor([[-0.2, 0.8], [2.8, 3.8]]),
+            torch.tensor(
+                [
+                    [[-3.0, -2.9], [-2.8, -2.9]],
+                    [[-2.7, -2.6], [-2.8, -2.9]],
+                ]
+            ),
+            torch.tensor([[-0.4], [0.6]]),
+        )
+        self._ablation_test_assert(
+            ablation_algo,
+            (inp1, inp2, inp3),
+            expected,
+            target=None,
+            baselines=(0.2, 3.1, 0.4),
+            test_enable_cross_tensor_attribution=[False, True],
+        )
+
     def test_multi_input_ablation_with_mask_weighted(self) -> None:
         ablation_algo = FeatureAblation(BasicModel_MultiLayer_MultiInput())
         ablation_algo.use_weights = True