Add Functional Interface for MeanAveragePrecision

[RoyiAvital]

🚀 Feature

Currently the MeanAveragePrecision for detection task does not have Functional Interface.

It would be great to have one.

Motivation

Same motivation as given in the Module vs Functional Metrics section from documentation.

It will be great to have such case as optimized as it can get.

[github-actions]

Hi! thanks for your contribution!, great first issue!

[Isalia20]

@Borda
I can take this and probably will have something by end of the week~

[Isalia20]

@Borda
Not sure if this is the best place to ask questions, I see that there are 2 MeanAveragePrecision implementations(2 classes), one in src/torchmetrics/detection/mean_ap.py and another src/torchmetrics/detection/_mean_ap.py

What is the difference between these two?

[Borda]

What is the difference between these two?

We started with the original package and made just a wrapper. Then we wanted to have own implementation which we could optimize for PyTorch, but it was still slower so we preserve the original and default until we improve the own code...