Fix Binary Stochastic Gates pyre fixme issues (#1600)

Summary:
Pull Request resolved: #1600

Fixing unresolved pyre fixme issues in corresponding file

Reviewed By: cyrjano

Differential Revision: D76737762

fbshipit-source-id: 24d648c232b34d9b5241a8b56d06e0e9c53f21a8

Author: Vivek Miglani

captum/module/binary_concrete_stochastic_gates.py

@@ -134,8 +134,9 @@ def __init__(
         self.eps = eps
 
         # pre-calculate the fixed term used in active prob
-        # pyre-fixme[4]: Attribute must be annotated.
-        self.active_prob_offset = temperature * math.log(-lower_bound / upper_bound)
+        self.active_prob_offset: float = temperature * math.log(
+            -lower_bound / upper_bound
+        )
 
     def _sample_gate_values(self, batch_size: int) -> Tensor:
         """