Make torchhub more robust regarding the master -> main renaming in other repos

[NicolasHug]

Note: just to clarify, we'll probably have to udpate the https://github.com/pytorch/hub repo to use main instead of master soon, but this issue isn't about this.

---

A lot of repos are moving away from the master branch name in favour of the main branch (torchaudio, torchvision, etc.)

Currently, torhhub assumes that the default branch of a repo is the master branch:

https://github.com/pytorch/pytorch/blob/726fd26b3e5ecf205569e59cf7cd5a6cbf4387a0/torch/hub.py#L107:L113

This can be problematic because for repos which have transitioned from master to main, the master branch will eventually become outdated, and the models that get downloaded with the default parameters to torch.hub.load() will be outdated too. Some repos may even delete their master branch.

I think we could change the default branch assumption to the following:

if main exists then use main, else use master

This new assumption might break for repos where the main branch exists while the development actually happens on master, but IMHO this isn't a common-enough scenario to worry about it.

If there are no concerns, I'll try to submit a PR for this next week unless someone else does it (which you are more than welcome to)

cc @nairbv @NicolasHug

[fmassa]

I agree with the proposal, and I think it will make a lot of things easier.