Fix Lime output dimension in batch forward (#1513)

Summary:

Currently, when a batch of inputs is provided with a forward function that returns a single scalar per batch, Lime and KernelShap still return output matching the input shape.

This behavior is inconsistent with other perturbation based methods, particularly Feature Ablation and Shapley Value Sampling.

This change breaks backward compatibility for OSS users, but since it's a specific case (scalar per batch), should be fine to update with only a documentation update.

Reviewed By: craymichael

Differential Revision: D70096644

Author: Vivek Miglani

Diff for: captum/attr/_core/lime.py

@@ -1038,7 +1038,12 @@ def attribute(  # type: ignore
                         coefficient of the corresponding interpretale feature.
                         All elements with the same value in the feature mask
                         will contain the same coefficient in the returned
-                        attributions. If return_input_shape is False, a 1D
+                        attributions.
+                        If forward_func returns a single element per batch, then the
+                        first dimension of each tensor will be 1, and the remaining
+                        dimensions will have the same shape as the original input
+                        tensor.
+                        If return_input_shape is False, a 1D
                         tensor is returned, containing only the coefficients
                         of the trained interpreatable models, with length
                         num_interp_features.
@@ -1242,6 +1247,7 @@ def _attribute_kwargs(  # type: ignore
                 coefs,
                 num_interp_features,
                 is_inputs_tuple,
+                leading_dim_one=(bsz > 1),
             )
         else:
             return coefs
@@ -1254,6 +1260,7 @@ def _convert_output_shape(
         coefs: Tensor,
         num_interp_features: int,
         is_inputs_tuple: Literal[True],
+        leading_dim_one: bool = False,
     ) -> Tuple[Tensor, ...]: ...
 
     @typing.overload
@@ -1264,6 +1271,7 @@ def _convert_output_shape(  # type: ignore
         coefs: Tensor,
         num_interp_features: int,
         is_inputs_tuple: Literal[False],
+        leading_dim_one: bool = False,
     ) -> Tensor: ...
 
     @typing.overload
@@ -1274,6 +1282,7 @@ def _convert_output_shape(
         coefs: Tensor,
         num_interp_features: int,
         is_inputs_tuple: bool,
+        leading_dim_one: bool = False,
     ) -> Union[Tensor, Tuple[Tensor, ...]]: ...
 
     def _convert_output_shape(
@@ -1283,6 +1292,7 @@ def _convert_output_shape(
         coefs: Tensor,
         num_interp_features: int,
         is_inputs_tuple: bool,
+        leading_dim_one: bool = False,
     ) -> Union[Tensor, Tuple[Tensor, ...]]:
         coefs = coefs.flatten()
         attr = [
@@ -1295,4 +1305,7 @@ def _convert_output_shape(
                     coefs[single_feature].item()
                     * (feature_mask[tensor_ind] == single_feature).float()
                 )
+        if leading_dim_one:
+            for i in range(len(attr)):
+                attr[i] = attr[i][0:1]
         return _format_output(is_inputs_tuple, tuple(attr))

Diff for: tests/attr/test_kernel_shap.py

@@ -348,9 +348,9 @@ def _multi_input_scalar_kernel_shap_assert(self, func: Callable) -> None:
         mask2 = torch.tensor([[0, 1, 2]])
         mask3 = torch.tensor([[0, 1, 2]])
         expected = (
-            [[3850.6666, 3850.6666, 3850.6666]] * 2,
-            [[306.6666, 3850.6666, 410.6666]] * 2,
-            [[306.6666, 3850.6666, 410.6666]] * 2,
+            [[3850.6666, 3850.6666, 3850.6666]],
+            [[306.6666, 3850.6666, 410.6666]],
+            [[306.6666, 3850.6666, 410.6666]],
         )
 
         self._kernel_shap_test_assert(

Diff for: tests/attr/test_lime.py

@@ -494,9 +494,9 @@ def _multi_input_scalar_lime_assert(self, func: Callable) -> None:
         mask2 = torch.tensor([[0, 1, 2]])
         mask3 = torch.tensor([[0, 1, 2]])
         expected = (
-            [[3850.6666, 3850.6666, 3850.6666]] * 2,
-            [[305.5, 3850.6666, 410.1]] * 2,
-            [[305.5, 3850.6666, 410.1]] * 2,
+            [[3850.6666, 3850.6666, 3850.6666]],
+            [[305.5, 3850.6666, 410.1]],
+            [[305.5, 3850.6666, 410.1]],
         )
 
         self._lime_test_assert(