best.pt not saved when it is from the last epoch

[AlexWang1900]

Before submitting a bug report, please be aware that your issue must be reproducible with all of the following, otherwise it is non-actionable, and we can not help you:

• Current repo: run git fetch && git status -uno to check and git pull to update repo
• Common dataset: coco.yaml or coco128.yaml
• Common environment: Colab, Google Cloud, or Docker image. See https://github.com/ultralytics/yolov5#reproduce-our-environment

If this is a custom dataset/training question you must include your train*.jpg, test*.jpg and results.png figures, or we can not help you. You can generate these with utils.plot_results().

🐛 Bug

in train.py line: 390
```
if (best_fitness == fi) and not final_epoch: ###
torch.save(ckpt, best)
del ckpt

when the last epoch happens to have a best fitness,  it wouldn't save a best.pt,
the code outside the train.py cannot judge which .pt  is the best.  


## To Reproduce (REQUIRED)

train one epoch from an interrupted traning's  last.pt , or the best result happens to be in the last epoch.

Output:
"couldn't find best.pt" or load a wrong best.pt. (should be last.pt in this situation)

## Expected behavior
A clear and concise description of what you expected to happen.
when training compeletes, one can load best.pt as it is the best result.


## Environment
If applicable, add screenshots to help explain your problem.

 - OS: [e.g. Ubuntu]
 - GPU [e.g. 2080 Ti]


## Additional context
Add any other context about the problem here.

[glenn-jocher]

@AlexWang1900 yes, good point! I do believe you've found a bug. Would you like to submit a PR or should I include this in the next push?

[AlexWang1900]

I'd Like to summit a PR!

…

---Original--- From: "Glenn Jocher"<notifications@github.com&gt; Date: Wed, Jul 29, 2020 02:21 AM To: "ultralytics/yolov5"<yolov5@noreply.github.com&gt;; Cc: "Mention"<mention@noreply.github.com&gt;;"AlexWang1900"<41888506@qq.com&gt;; Subject: Re: [ultralytics/yolov5] best.pt not saved when it is from the last epoch (#542) @AlexWang1900 yes, good point! I do believe you've found a bug. Would you like to submit a PR or should I include this in the next push? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub, or unsubscribe.

[glenn-jocher]

Issue resolved, removing TODO.