Type signature fix for private methods of qMultiStepLookahead

Summary: `posterior_transform` is `Optional` in the constructor of the acquisition function, so it should be optional in the private methods as well.

Reviewed By: saitcakmak

Differential Revision: D48244486

fbshipit-source-id: f0850ecde5259d626cacbacd8c36401483a69d15

botorch/acquisition/multi_step_lookahead.py

@@ -322,7 +322,7 @@ def _step(
     valfunc_argfacs: List[Optional[TAcqfArgConstructor]],
     inner_samplers: List[Optional[MCSampler]],
     objective: MCAcquisitionObjective,
-    posterior_transform: PosteriorTransform,
+    posterior_transform: Optional[PosteriorTransform],
     running_val: Optional[Tensor] = None,
     sample_weights: Optional[Tensor] = None,
     step_index: int = 0,
@@ -427,7 +427,7 @@ def _compute_stage_value(
     valfunc_cls: Optional[Type[AcquisitionFunction]],
     X: Tensor,
     objective: MCAcquisitionObjective,
-    posterior_transform: PosteriorTransform,
+    posterior_transform: Optional[PosteriorTransform],
     inner_sampler: Optional[MCSampler] = None,
     arg_fac: Optional[TAcqfArgConstructor] = None,
 ) -> Optional[Tensor]: