Fix pyre errors in InputXGradient (pytorch#1397)

Summary:

Initial work on fixing Pyre errors in InputXGradient

Reviewed By: csauper

Differential Revision: D64677348

captum/attr/_core/input_x_gradient.py

@@ -11,6 +11,7 @@
 from captum._utils.typing import TargetType, TensorOrTupleOfTensorsGeneric
 from captum.attr._utils.attribution import GradientAttribution
 from captum.log import log_usage
+from torch import Tensor
 
 
 class InputXGradient(GradientAttribution):
@@ -20,8 +21,7 @@ class InputXGradient(GradientAttribution):
     https://arxiv.org/abs/1605.01713
     """
 
-    # pyre-fixme[24]: Generic type `Callable` expects 2 type parameters.
-    def __init__(self, forward_func: Callable) -> None:
+    def __init__(self, forward_func: Callable[..., Tensor]) -> None:
         r"""
         Args:
 
@@ -35,8 +35,7 @@ def attribute(
         self,
         inputs: TensorOrTupleOfTensorsGeneric,
         target: TargetType = None,
-        # pyre-fixme[2]: Parameter annotation cannot be `Any`.
-        additional_forward_args: Any = None,
+        additional_forward_args: object = None,
     ) -> TensorOrTupleOfTensorsGeneric:
         r"""
         Args:
@@ -113,28 +112,20 @@ def attribute(
         """
         # Keeps track whether original input is a tuple or not before
         # converting it into a tuple.
-        # pyre-fixme[6]: For 1st argument expected `Tensor` but got
-        #  `TensorOrTupleOfTensorsGeneric`.
         is_inputs_tuple = _is_tuple(inputs)
 
-        # pyre-fixme[9]: inputs has type `TensorOrTupleOfTensorsGeneric`; used as
-        #  `Tuple[Tensor, ...]`.
-        inputs = _format_tensor_into_tuples(inputs)
-        # pyre-fixme[6]: For 1st argument expected `Tuple[Tensor, ...]` but got
-        #  `TensorOrTupleOfTensorsGeneric`.
-        gradient_mask = apply_gradient_requirements(inputs)
+        inputs_tuple = _format_tensor_into_tuples(inputs)
+        gradient_mask = apply_gradient_requirements(inputs_tuple)
 
         gradients = self.gradient_func(
-            self.forward_func, inputs, target, additional_forward_args
+            self.forward_func, inputs_tuple, target, additional_forward_args
         )
 
         attributions = tuple(
-            input * gradient for input, gradient in zip(inputs, gradients)
+            input * gradient for input, gradient in zip(inputs_tuple, gradients)
         )
 
-        # pyre-fixme[6]: For 1st argument expected `Tuple[Tensor, ...]` but got
-        #  `TensorOrTupleOfTensorsGeneric`.
-        undo_gradient_requirements(inputs, gradient_mask)
+        undo_gradient_requirements(inputs_tuple, gradient_mask)
         # pyre-fixme[7]: Expected `TensorOrTupleOfTensorsGeneric` but got
         #  `Tuple[Tensor, ...]`.
         return _format_output(is_inputs_tuple, attributions)