Make DummyKernelDistribution actually inherit from Distribution

[rg936672]

What's the issue?

Currently, InertKernelModel returns a DummyKernelDistribution when called. However, its superclass (ExactGPModel) returns a MultivariateNormal, and DummyKernelDistribution is not a subclass of MultivariateNormal or even of Distribution. This violates the Liskov substitution principle.

Implement all required methods to make DummyKernelDistribution inherit from Distribution, and change the type hint on ExactGPModel.forward to indicate that it returns a Distribution (Possibly also change other code downstream of this to reflect this change as well.)