Removal schedule for legacy model builder API

Although our documentation is explicit

https://github.com/pytorch/vision/blob/a05d81796dd4743e8b16797a43e9166f0cdb90c7/docs/source/models.rst?plain=1#L82

our runtime warnings are really vague:

• https://github.com/pytorch/vision/blob/ecbed7bd604a705d4b3709d97817598166b480ca/torchvision/models/_utils.py#L135-L139
• https://github.com/pytorch/vision/blob/ecbed7bd604a705d4b3709d97817598166b480ca/torchvision/models/_utils.py#L208-L211
• https://github.com/pytorch/vision/blob/ecbed7bd604a705d4b3709d97817598166b480ca/torchvision/models/_utils.py#L213-L217

Apart from not committing to an explicit removal date, we are not even committing to removing this in general, i.e.

may be removed

In https://github.com/pytorch/vision/pull/7176#discussion_r1098367538 we decided to not remove this functionality together with all other scheduled removals for 0.15.

However, we should decide on the future of this. Here are some options that I see:

1. Make the date in the runtime warnings explicit and remove afterwards. Abiding by our own BC policy, we probably need to use 0.17 as removal date. At that point, users saw the warning for 4 releases and thus roughly 1 year, which gave them plenty time to migrate.
2. Remove the documentation of the legacy API, but keep supporting it silently, i.e. no warnings. Going this route would mean that new users won't know about the legacy API, but old code doesn't need to be updated.
3. Keep everything as is.

My order of preference is 1. > 2. > 3.

Start by reading the legacy API documentation in docs/source/models.rst and the related warning paths in torchvision/models/_utils.py. Review the BC policy and the discussion in pull request 7176. Done means reaching a decision on the legacy API's future and updating the documented removal schedule and runtime warnings accordingly.