Pin scipy to <1.15 (again) to avoid slowdowns (#2669)

Summary:
There appear to be substantial slowdowns with the optimization (presumably both for fitting and candidate generation) with scipy 1.15: #2668

This pins the scipy version to <1.15 for now to avoid these slowdowns and terrible user experiences (at least on the `main` branch) until we've figured out the root cause of these slowdowns.

Pull Request resolved: #2669

Reviewed By: saitcakmak

Differential Revision: D67826874

Pulled By: Balandat

fbshipit-source-id: 9562cb8ea8f25cc51165931fa4335191e6e4b34c

requirements.txt

@@ -4,5 +4,5 @@ gpytorch==1.13
 linear_operator==0.5.3
 torch>=2.0.1
 pyro-ppl>=1.8.4
-scipy
+scipy<1.15
 multipledispatch