add/move a few apis in torch.hub (#18758)
Summary:
* `torch.hub.list('pytorch/vision')` - show all available hub models in `pytorch/vision`
* `torch.hub.show('pytorch/vision', 'resnet18')` - show docstring & example for `resnet18` in `pytorch/vision`
* Moved `torch.utils.model_zoo.load_url` to `torch.hub.load_state_dict_from_url` and deprecate `torch.utils.model_zoo`
* We have too many env to control where the cache dir is, it's not very necessary. I actually want to unify `TORCH_HUB_DIR`, `TORCH_HOME` and `TORCH_MODEL_ZOO`, but haven't done it. (more suggestions are welcome!)
* Simplify `pytorch/vision` example in doc, it was used to show how how hub entrypoint can be written so had some confusing unnecessary args.
An example of hub usage is shown below
```
In [1]: import torch
In [2]: torch.hub.list('pytorch/vision', force_reload=True)
Downloading: "https://github.com/pytorch/vision/archive/master.zip" to /private/home/ailzhang/.torch/hub/master.zip
Out[2]: ['resnet18', 'resnet50']
In [3]: torch.hub.show('pytorch/vision', 'resnet18')
Using cache found in /private/home/ailzhang/.torch/hub/vision_master
Resnet18 model
pretrained (bool): a recommended kwargs for all entrypoints
args & kwargs are arguments for the function
In [4]: model = torch.hub.load('pytorch/vision', 'resnet18', pretrained=True)
Using cache found in /private/home/ailzhang/.torch/hub/vision_master
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18758
Differential Revision: D14883651
Pulled By: ailzhang
fbshipit-source-id: 6db6ab708a74121782a9154c44b0e190b23e8309
