Fix typing error in stochastic gates base (#1584)

Summary:
Pull Request resolved: #1584

Fixes https://github.com/pytorch/captum/actions/runs/15593782761/job/43919109271

Reviewed By: sarahtranfb

Differential Revision: D76604756

fbshipit-source-id: 94fa39c4c0eef6cd69cacca4de87b50d306204e1

Author: craymichael

captum/module/stochastic_gates_base.py

@@ -30,6 +30,8 @@ class StochasticGatesBase(Module, ABC):
     extend this class and implement the distribution specific functions.
     """
 
+    mask: Optional[Tensor]
+
     def __init__(
         self,
         n_gates: int,