Replace calls to deprecated mvn argument of GPyTorchPosterior wit…

…h `distribution` (#2289)

Summary:

This will not change behavior.

Reviewed By: saitcakmak

Differential Revision: D55094724

ax/models/torch/alebo.py

@@ -398,7 +398,7 @@ def posterior(
         assert output_indices is None
         assert not observation_noise
         mvn = self(X)
-        posterior = GPyTorchPosterior(mvn=mvn)
+        posterior = GPyTorchPosterior(distribution=mvn)
         if posterior_transform is not None:
             return posterior_transform(posterior)
         return posterior