

TITLE
RAG Retriever  (NameError: name 'load_dataset' is not defined   in retrieval_rag.py) 

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## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 3.3.0
- Platform: Linux-4.19.0-11-cloud-amd64-x86_64-with-debian-10.6
- Python version: 3.7.3
- PyTorch version (GPU?): 1.6.0+cpu (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: NO
- Using distributed or parallel set-up in script?: NO

### Who can help 
@sshleifer
RAG model is not on the list, but this is summarization related
 -->

## Information

Model I am using RAG

The problem arises when using:
* [ +] the official example scripts: (give details below)
``` python from transformers import RagTokenizer, RagRetriever, RagSequenceForGeneration
import torch

tokenizer = RagTokenizer.from_pretrained("facebook/rag-sequence-nq")
retriever = RagRetriever.from_pretrained("facebook/rag-sequence-nq", index_name="exact", use_dummy_dataset=True)
# initialize with RagRetriever to do everything in one forward call
model = RagSequenceForGeneration.from_pretrained("facebook/rag-token-nq", retriever=retriever) 
```


The tasks I am working on is:
Model coudln't load, didn't perform any task

## To reproduce

Steps to reproduce the behavior:

1. run the code
``` python from transformers import RagTokenizer, RagRetriever, RagSequenceForGeneration
import torch

tokenizer = RagTokenizer.from_pretrained("facebook/rag-sequence-nq")
retriever = RagRetriever.from_pretrained("facebook/rag-sequence-nq", index_name="exact", use_dummy_dataset=True)
# initialize with RagRetriever to do everything in one forward call
model = RagSequenceForGeneration.from_pretrained("facebook/rag-token-nq", retriever=retriever) 
```


## Expected behavior

Raise a NameError, load_dataset is not defined.

```python
NameError                                 Traceback (most recent call last)
<ipython-input-6-752205d4a1c8> in <module>
      3 
      4 tokenizer = RagTokenizer.from_pretrained("facebook/rag-sequence-nq")
----> 5 retriever = RagRetriever.from_pretrained("facebook/rag-sequence-nq", index_name="exact", use_dummy_dataset=True)
      6 # initialize with RagRetriever to do everything in one forward call
      7 model = RagSequenceForGeneration.from_pretrained("facebook/rag-token-nq", retriever=retriever)

/mnt/disks/nlp/env_nlp_main/lib/python3.7/site-packages/transformers/retrieval_rag.py in from_pretrained(cls, retriever_name_or_path, **kwargs)
    307         generator_tokenizer = rag_tokenizer.generator
    308         return cls(
--> 309             config, question_encoder_tokenizer=question_encoder_tokenizer, generator_tokenizer=generator_tokenizer
    310         )
    311 

/mnt/disks/nlp/env_nlp_main/lib/python3.7/site-packages/transformers/retrieval_rag.py in __init__(self, config, question_encoder_tokenizer, generator_tokenizer)
    287                 config.retrieval_vector_size,
    288                 config.index_path,
--> 289                 config.use_dummy_dataset,
    290             )
    291         )

/mnt/disks/nlp/env_nlp_main/lib/python3.7/site-packages/transformers/retrieval_rag.py in __init__(self, dataset_name, dataset_split, index_name, vector_size, index_path, use_dummy_dataset)
    218 
    219         logger.info("Loading passages from {}".format(self.dataset_name))
--> 220         self.dataset = load_dataset(
    221             self.dataset_name, with_index=False, split=self.dataset_split, dummy=self.use_dummy_dataset
    222         )

NameError: name 'load_dataset' is not defined
```
