Supporting `config_path` for `AutoModel`
# 🚀 Feature request

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Creating a model like this:
```python
AutoModel.from_pretrained('https://myserver/model/pytorch_model.bin', config='path/to/local/config.json') 
```
## Motivation

For the moment, instantiating a model like above is possible with `PretrainedModel` but not with `AutoModel`. i.e.
```python
BertModel.from_pretrained('https://myserver/model/pytorch_model.bin', config='path/to/local/config.json') # possible
AutoModel.from_pretrained('https://myserver/model/pytorch_model.bin', config='path/to/local/config.json') # not possible
```

To elaborate, the optional `config` argument passed to **`AutoModel.from_pretrained`** method should be `PretrainedConfig`, while it could be either `PretrainedConfig` or 'a string or path valid as input to `PretrainedConfig.from_pretrained`' in the case of **`PretrainedModel.from_pretrained`**.

The difference comes from the lack of [this line](https://github.com/huggingface/transformers/blob/8780caa388c7b2aa937454ed96bcdd3f097f851d/src/transformers/modeling_utils.py#L974) in `AutoModel`.



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## Your contribution
I would like to submit a PR.
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Hey @hwijeen! Would the `AutoModel.from_config()` method work for you?<|||||>Thank you @LysandreJik for your quick reply! I checked out the `AutoModel.from_config()` method. It is a convenient method but does not fit my use case, as **it does NOT load weights**.<|||||>Hi @LysandreJik , I update this issue to clarify. I would appreciate if you could give it a pass! I am sorry to nudge you twice but I think this issue and related PR(#10981) could be useful for those who try to load pretrained weights from private server!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.