Replace plain asserts with ValueErrors in bacbone utils

[oke-aditya]

As noted in #5209 (comment)

It would be great if these can be replaced by ValueErrors.

https://github.com/pytorch/vision/blob/main/torchvision/models/detection/backbone_utils.py#L114
https://github.com/pytorch/vision/blob/main/torchvision/models/detection/backbone_utils.py#L127

Maybe these too?
https://github.com/pytorch/vision/blob/main/torchvision/models/detection/backbone_utils.py#L155
https://github.com/pytorch/vision/blob/main/torchvision/models/detection/backbone_utils.py#L186

Once the assert is replaced by valueError. We should add test to check for it.
Probably here
https://github.com/pytorch/vision/blob/main/test/test_backbone_utils.py

cc @datumbox @abhi-glitchhg

[oke-aditya]

Thanks a lot for sending PRs! It's great you guys are interested 😃
Hi! I see there are 2 PRs. @piyush01123 and @abhi-glitchhg can you guys sync a bit so that we can avoid duplication? Maybe work together and come up with tests?

[abhi-glitchhg]

yes Sure, @piyush01123 what is your thought?
I have added tests for resnet_backbone, and am yet to add for the mobilenet_backbone; if you want to do that part let me know; we can merge our work in a single pr later.

[piyush01123]

I've created new PR #5275 which contains @abhi-glitchhg 's commits from earlier and my commits for mobilenet test case.