Update VariFocal Loss Function

[Sangh0]

I added vari focal loss function in utils/loss.py
Also, I added other some codes to calculate vari focal loss.
I experimented with vari focal loss, its performance is so good.
So,I want you refer to this PR.
Thank you.

[xoxograce15]

@Sangh0 So how do i ensure that my program is using varifocalloss function? What changes do I need to do, inorder for me to utilize varifocal and not CIOU, DIOU, etc. as in #1721 (comment)
If you could please help me, I'd be very grateful. Thanks

[Sangh0]

@xoxograce15 Varifocal loss is a classification loss and not bounding box loss. If you want to train model with varifocal loss, you need to implement varifocal loss. Also, if you need references, visiting my repository

[xoxograce15]

Thank you for your reply. Yes, I want to train model with varifocal loss, but how exactly are you suggesting me to "implement varifocal loss". I didn't get you. Like, do i need to do some changes in the loss.py or how?
I am already following the repository you linked https://github.com/Sangh0/yolov7_varifocal
Any information would be very helpful for my research work as I am working around this topic so. Thanks <3

[Sangh0]

@xoxograce15 Sure, here varifocal loss in loss.py is the codes that you want. The code from line 177 to line 229 and line 498 is for varifocal loss.