[jit] Add return statement back to Future::addCallback() (#36662)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36662

This was a mistake from an earlier change, though the expected impact is
relatively minimal - mostly keeping callback around longer than necessary
in the case of callbacks already-completed futures.
ghstack-source-id: 102203224

Test Plan: buck test mode/dev-nosan caffe2/test/...

Differential Revision: D21044145

fbshipit-source-id: f3bd58bd6bde83caaa7b9bd0385d0ce3647dbc05
diff --git a/torch/csrc/utils/future.h b/torch/csrc/utils/future.h
index 0949718..82507da 100644
--- a/torch/csrc/utils/future.h
+++ b/torch/csrc/utils/future.h
@@ -124,6 +124,7 @@
     if (completed_) {
       lock.unlock();
       cb();
+      return;
     }
     callbacks_.emplace_back(std::move(cb));
   }