Bug Report In _evaluate_fitness function

[gxlover0625]

Hello, thanks to your wonderful work PromptBreeder in prompt-optimization area.
When I ran the code, I felt confused about the _evaluate_fitness function. After carefully reviewing the code, I found that the following part has issues.

PromptBreeder/pb/__init__.py

Lines 129 to 135 in 637ea25

	if unit.fitness > elite_fitness:
	# I am copying this bc I don't know how it might get manipulated by future mutations.
	unit = population.units[unit_index]
	current_elite = unit.model_copy()
	elite_fitness = unit.fitness

I found others' questions about this part in the issue and submitted a PR. #1
However, I noticed that the change is incorrect, since unit, being a local variable, is referenced before declaration. It first appears in the for loop on line 102.

PromptBreeder/pb/__init__.py

Line 102 in 637ea25

for unit in population.units:

To obtain the best elites, all instances of unit should be replaced with population.units[unit_index].
Here is my code:

if population.units[unit_index].fitness > elite_fitness:
    current_elite = population.units[unit_index].model_copy()
    elite_fitness = population.units[unit_index].fitness

I believe my changes may address the issue correctly and follow the original paper’s design. If you agree, I’d be happy to submit a PR. Your feedback is greatly appreciated.