This issue proposes to separate indexing logic from distribution logic in the Categorical distribution.

Currently Pyro's Categorical distribution bakes in logic to index against a list of values. This design choice leads to a number of difficulties:

• Categorical is difficult to test and requires lots of custom testing logic.
• It is difficult to migrate to torch.distributions whose categorical works with LongTensors.

Support for vs may be added back in later via a discrete Bijector.