Add option to choose the loss function in `compute_loss` function.

[abhi-glitchhg]

🚀 The feature

In the compute loss function, giou_loss is used to calculate the loss by default see here, but now as CIOU and DIOU loss functions are added to torchvision, maybe we could let the user choose in between the loss functions

cc @datumbox @YosuaMichael, @NicolasHug,

Motivation, pitch

This gives little more freedom to the user to choose between the loss functions.

Alternatives

No response

Additional context

No response

[datumbox]

@abhi-glitchhg It seems you've uncovered my evil plan... 😄

The reason why I've asked you to add ciou/diou was to test them on RetinaNet. This functionality was already added on the specific model (see #5756) but it's private because we don't want to commit to the API yet. I've kicked off a PR to experiment with it at #5984 and see if it brings improvements. It doesn't do exactly what you want cause it's not on FCOS but it sets the basis.