Add activation function option for yolov8 detector.

[digital-nomad-cheng]

What does this PR do?

Currently, only YOLOV8Backbon accept activation as argument for changing to other activation function other than swish. But for some hardware, swish activation function is not well optimized. It does not make sense to only change the backbone to other activation function while keep the detector(head) default to swish. So this pull meant to give option to choose activation in detector as well.

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[digital-nomad-cheng]

@divyashreepathihalli Can you help have a look at this PR? Thanks!

[divyashreepathihalli]

Thanks for the PR. There was recent changes to YOLV8. Can you please rebase and also runn the shell/format.sh at root to clear formatting errors

[digital-nomad-cheng]

Thanks for the PR. There was recent changes to YOLV8. Can you please rebase and also runn the shell/format.sh at root to clear formatting errors

Done