[TorchHub]Repo's layout is not compatible with TorchHub anymore since 2.0
# 🐛 Bug

When I try loading a model/tokenizer from the [pytorch hub](https://pytorch.org/hub/huggingface_pytorch-transformers/) page, the hub loading code is not working anymore. Pre transformers 2.0.0, the same loading code is working.

On a quick look, I believe it's related to the repo's fold layout since 2.0 where the `transformers` module is moved inside `src` but `hub_conf` is still assuming `transformers` exists in the same level and we get a module not found error.  For context, torch hub insert the repo root directory into `sys.path` to enable import.

## To reproduce

Run
```
import torch
tokenizer = torch.hub.load('huggingface/transformers:v2.5.0', 'tokenizer', 'bert-base-cased')
```
Stack trace:

```
ModuleNotFoundError                       Traceback (most recent call last)
<ipython-input-5-155f4aa294f1> in <module>
      1 import torch
----> 2 tokenizer = torch.hub.load('huggingface/transformers:v2.5.0', 'tokenizer', 'bert-base-cased')
      3 
      4 text_1 = "Who was Jim Henson ?"
      5 text_2 = "Jim Henson was a puppeteer"

~/miniconda3/envs/poc/lib/python3.7/site-packages/torch/hub.py in load(github, model, *args, **kwargs)
    354     sys.path.insert(0, repo_dir)
    355 
--> 356     hub_module = import_module(MODULE_HUBCONF, repo_dir + '/' + MODULE_HUBCONF)
    357 
    358     entry = _load_entry_from_hubconf(hub_module, model)

~/miniconda3/envs/poc/lib/python3.7/site-packages/torch/hub.py in import_module(name, path)
     70         spec = importlib.util.spec_from_file_location(name, path)
     71         module = importlib.util.module_from_spec(spec)
---> 72         spec.loader.exec_module(module)
     73         return module
     74     elif sys.version_info >= (3, 0):

~/miniconda3/envs/poc/lib/python3.7/importlib/_bootstrap_external.py in exec_module(self, module)

~/miniconda3/envs/poc/lib/python3.7/importlib/_bootstrap.py in _call_with_frames_removed(f, *args, **kwds)

~/.cache/torch/hub/huggingface_transformers_v2.5.0/hubconf.py in <module>
----> 1 from transformers import (
      2     AutoConfig,
      3     AutoModel,
      4     AutoModelForQuestionAnswering,
      5     AutoModelForSequenceClassification,

ModuleNotFoundError: No module named 'transformers'
```

## Expected behavior

This was working before 2.0.0. 

```
import torch
tokenizer = torch.hub.load('huggingface/transformers:1.2.0', 'tokenizer', 'bert-base-cased')

```

## Environment info
     
- `transformers` version: >2.0.0
- Platform: all
- Python version: 3.7
- PyTorch version (GPU?): 1.3.1
- Tensorflow version (GPU?): N/A
- Using GPU in script?: N/A
- Using distributed or parallel set-up in script?: N/A

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>Any plan to fix this?<|||||>I missed that issue, but this was fixed a couple of weeks ago, and it's even covered by CI now: https://github.com/huggingface/transformers/blob/master/.github/workflows/github-torch-hub.yml