Generates the initial population and assigns fitnesses.

def _initialize_pop(self, pop_size):
        """Generates the initial population and assigns fitnesses."""
        self.initialize_cma_es(pop_size)
        self.toolbox.register("individual", self._make_individual)
        self.toolbox.register("generate", self._generate,
                              self.toolbox.individual)
        self.toolbox.register("population", tools.initRepeat,
                              list, self._initial_individual)
        self.toolbox.register("update", self.update)
        self.population = self.toolbox.population(n=pop_size)
        self.assign_fitnesses(self.population)
        self._model_count += len(self.population)
        return