Add typing in detection module (faster rcnn).

[khushi-411]

Follows #4582.

[oke-aditya]

Hi @khushi-411.

Nice to see the PR,

Faster-rcnn file depends on other files currently in the chain.
Since we inherit from generalized rcnn, and also need transforms to be typed. It is good to wait a while.
I suggest to not close this PR, but instead just wait as we work up the chain 😃 Surely we will make use of these types soon!

[khushi-411]

Hi @oke-aditya,
Thanks, it was a pleasant start after a week's holiday.

BTW, Thanks for the info!

[datumbox]

FYI there is a parallel effort of refactoring models inside the prototype folder. See:
https://github.com/pytorch/vision/blob/main/torchvision/prototype/models/detection/faster_rcnn.py

I believe this one is typed but worth checking. :)

[oke-aditya]

Eventhough the CI looks green, let's not hurry 😃 Since inherited classes can often mismatch later.