Update the forecasting with the usage of Predictive

[fehiepsi]

Originally, we wrote the tutorial to introduce some handlers and the usage of vmap to forecast values. But for many cases, we don't need to learn about handlers to use NumPyro. So this PR adds a small section to illustrate that we can use Predictive utility to get the same result with just 2 lines of code.

[neerajprad]

This looks good. If we have introduced effect handlers in other tutorials, I am fine with even removing the whole forecasting part and using Predictive by default if that keeps things simple.

[fehiepsi]

Yeah, you are right. We already covered handlers in regression tutorial. Let me remove that part.