Raise warning for the automatic enumerate behavior

[pookiecoding]

Following a thread on NumPyro forum about inference with discrete latent variable in hmm, here is the github issue as requested:
In the hmm example, the discrete latent variable are marginalized out automatically.
To avoid confusion, in future versions, the "automatic" behavior could be removed, and the users would provide infer={"enumerate": "parallel"} explicitly.
Thank you.