support passing path to a `config` variable in AutoClass
Author: hwijeen
Pull request: yes

# What does this PR do?

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It enables loading weights from a private server like the following.
```python
AutoModel.from_pretrained('https://myserver/model/pytorch_model.bin', config='path/to/local/config.json') 
```
**For the moment, the function to load weights from a private server like this way is supported in `PretrainedModel` but not in `AutoModel`.**
```python
BertModel.from_pretrained('https://myserver/model/pytorch_model.bin', config='path/to/local/config.json') # supported
AutoModel.from_pretrained('https://myserver/model/pytorch_model.bin', config='path/to/local/config.json') # not supported
```
To fix this issue, I copy and pasted code about `config_path` from `PretrainedModel` to `AutoModel`.

This features was requested in #10961 .

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