Reuse jinja environment for a prompt

[jantrienes]

I really like the @outlines.prompt decorator. However, I noticed it is slow when formatting a large number of examples. The main inefficiency is creating the Jinja environment for every call of render. This PR makes a minor change to re-use the same Jinja environment per prompt.

Note: This makes the render function obsolete. However, I kept it for backwards compatibility. Please advise if this should be further refactored.

Benchmark

Small benchmark to test it:

import timeit
import outlines

@outlines.prompt
def test_tpl(variable):
    """{{variable}} test"""

print('Formatted template:', test_tpl(variable='this is a'))

def test_render():
    test_tpl(variable='this is a')

execution_time = timeit.timeit(test_render, number=1000)
print(f"Execution time for 1000 runs: {execution_time} seconds")

With main:

Formatted template: this is a test
Execution time for 1000 runs: 0.11741420900216326 seconds

With this PR:

Formatted template: this is a test
Execution time for 1000 runs: 0.004638582991901785 seconds

[cpfiffer]

Big fan of this PR, free speed = dope.

Note: This makes the render function obsolete. However, I kept it for backwards compatibility. Please advise if this should be further refactored.

Could you add a comment or something to the render method so people are aware it's a no-op?

I'm also curious if anyone's actually using it at the lowered level. I suspect this group of people is relatively small, in which case just removing the method is probably fine with an appropriate deprecation warning.

We might also like to have tests using 1-2 different variables as well to test that this still works in both cases.

[rlouf]

Great PR! We are moving this logic to our new prompts library, and it would be awesome if you could submit a PR there as well!