[BugFix] Fix the performance issue of DeepACO

[Furffico]

Description

This pull request fixes the performance issue of the DeepACO implementation, and makes some tiny modifications to enhance the performance.

[hyeok9855]

Thank you for your work! I have added some comments.

[Furffico]

Made some modifications following the discussions above.
BTW, rl4co/models/zoo/deepaco/decoder.py seems redundant. Is it ok to delete this file?

[fedebotu]

I guess so, we had a look at that too. Right @hyeok9855 ?

[fedebotu]

@Furffico did the above fixes make DeepACO match the performance in your paper?

[Furffico]

@Furffico did the above fixes make DeepACO match the performance in your paper?

Yes. With a parallelized AS and a better pheromone update function, it's faster and sometimes perform better than our original implementation.

[fedebotu]

Yes. With a parallelized AS and a better pheromone update function, it's faster and sometimes perform better than our original implementation.

Good! Will the same hold with local search too @hyeok9855 ?

[hyeok9855]

LGTM
I will check the code more thoroughly later when I work for the local search #140 (maybe next week).
Thanks for your work!

[hyeok9855]

Good! Will the same hold with local search too @hyeok9855 ?

Yes, very likely.

[Furffico]

I think there's no further modifications required in this PR. Shall we merge it now?