Add eval method and chain function to compile outlines graphs

[rlouf]

We first provide a chain function which allow to compile outlines graph to an executable function:

import outlines
from outlines.text import string

s = string()
t = string()
o = s + " " + t
chain = outlines.chain([s, t], o)
print(chain("one", "two"))
# one two

We also add an eval method to Variables to make it easier to evaluate variables during debugging:

import outlines
from outlines.text import string

s = string()
t = string()
o = s + " " + t

print(o.eval({s: "one", t: "two"}))
# one two

In a future iteration we may want to rename chain to function and implement chain a decorator around a function instead:

import outlines

@outlines.chain
def add(s: str, t: str):
    return s + t

print(add("one", "two"))
# one two

import outlines

def add(s: str, t: str):
    return s + t

print(outlines.chain(add)("one", "two"))
# one two

We also add meta-prompting examples in the examples folder. Closes #1.