Implement Expect

[HEmile]

Implementing Expect is not as easy as I hoped. Unfortunately, in discrete VAEs, for example, you pass logits with shape [batch, latents, categories]. enumerate_support() does not recognize that batch is independent, but the different latents are dependent! It only enumerates the support over the categories dimension, but it should also enumerate over the latents dimension. To do this automatically however requires independency assumptions, which probably brings us back to Issue #4.

[HEmile]

Idea: Automatically assume all non-batched dimensions dependent, and enumerate over all dimensions (will probably throw an error). Figure out how to do that, probably possible. To make sure the batched dimension is ignored, maybe use storch.denote_independent(torch.tensor, dim) (in this case, storch.denote_independent(logits, 0)). See #4.