[RAG] RagSequenceForGeneration: Running "retriever separately example" giving error
## 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: 3.3.1
- Platform: Linux-4.19.112+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.6.0+cu101 (False)
- Tensorflow version (GPU?): 2.3.0 (False)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
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 albert, bert, GPT2, XLM: @LysandreJik 
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 Speed and Memory Benchmarks: @patrickvonplaten
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 Translation: @sshleifer
 Summarization: @sshleifer
 TextGeneration: @TevenLeScao 
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
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@patrickvonplaten @LysandreJik 

## Information

Model I am using (Bert, XLNet ...): RAG

The problem arises when using:
* [X] the official example scripts: (give details below)

The tasks I am working on is:
* [X] an official GLUE/SQUaD task: `dummy_dataset`

## To reproduce

Steps to reproduce the behavior:

1. Execute code snippets provided (Partially modified example script from https://huggingface.co/transformers/master/model_doc/rag.html)

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Code snippets:
```
!pip install git+https://github.com/huggingface/transformers.git
!pip install datasets
!pip install faiss-cpu

from transformers import RagTokenizer, RagRetriever, RagTokenForGeneration, RagSequenceForGeneration
import torch
import faiss

tokenizer = RagTokenizer.from_pretrained("facebook/rag-sequence-nq")
model = RagSequenceForGeneration.from_pretrained("facebook/rag-sequence-nq", use_dummy_dataset=True)
retriever = RagRetriever.from_pretrained("facebook/rag-sequence-nq", index_name="exact", use_dummy_dataset=True)

input_dict = tokenizer.prepare_seq2seq_batch("How many people live in Paris?", "In Paris, there are 10 million people.", return_tensors="pt")
input_ids = input_dict["input_ids"]

# Caling retriever seperately

question_hidden_states = model.question_encoder(input_ids)[0]
# 2. Retrieve
docs_dict = retriever(input_ids.numpy(), question_hidden_states.detach().numpy(), return_tensors="pt")
print(docs_dict)
doc_scores = torch.bmm(question_hidden_states.unsqueeze(1), docs_dict["retrieved_doc_embeds"].float().transpose(1, 2)).squeeze(1)
# 3. Forward to generator
outputs = model.generate(input_ids=input_ids, context_input_ids=docs_dict["context_input_ids"], context_attention_mask=docs_dict["context_attention_mask"], doc_scores=doc_scores)

generated_string = tokenizer.batch_decode(outputs, skip_special_tokens=True)
print(generated_string)

```
Stacktrace:
```
AssertionError                            Traceback (most recent call last)
<ipython-input-5-9f622b1f6353> in <module>()
      7 doc_scores = torch.bmm(question_hidden_states.unsqueeze(1), docs_dict["retrieved_doc_embeds"].float().transpose(1, 2)).squeeze(1)
      8 # 3. Forward to generator
----> 9 outputs = model.generate(input_ids=input_ids, context_input_ids=docs_dict["context_input_ids"], context_attention_mask=docs_dict["context_attention_mask"], doc_scores=doc_scores)
     10 generated_string = tokenizer.batch_decode(outputs, skip_special_tokens=True)
     11 print(generated_string)

5 frames
/usr/local/lib/python3.6/dist-packages/torch/autograd/grad_mode.py in decorate_context(*args, **kwargs)
     13         def decorate_context(*args, **kwargs):
     14             with self:
---> 15                 return func(*args, **kwargs)
     16         return decorate_context
     17 

/usr/local/lib/python3.6/dist-packages/transformers/modeling_rag.py in generate(self, input_ids, attention_mask, context_input_ids, do_deduplication, num_return_sequences, num_beams, **kwargs)
    902             # then, run model forwards to get nll scores:
    903             new_input_ids = input_ids[index : index + 1].repeat(len(output_sequences), 1)
--> 904             outputs = self(new_input_ids, labels=output_sequences, exclude_bos_score=True)
    905             top_cand_inds = (-outputs["loss"]).topk(num_doc_return_sequences)[1]
    906 

/usr/local/lib/python3.6/dist-packages/torch/nn/modules/module.py in _call_impl(self, *input, **kwargs)
    720             result = self._slow_forward(*input, **kwargs)
    721         else:
--> 722             result = self.forward(*input, **kwargs)
    723         for hook in itertools.chain(
    724                 _global_forward_hooks.values(),

/usr/local/lib/python3.6/dist-packages/transformers/modeling_rag.py in forward(self, input_ids, attention_mask, encoder_outputs, decoder_input_ids, decoder_attention_mask, past_key_values, context_input_ids, context_attention_mask, doc_scores, use_cache, output_attentions, output_hidden_states, output_retrieved, exclude_bos_score, reduce_loss, labels, **kwargs)
    767             output_attentions=output_attentions,
    768             output_hidden_states=output_hidden_states,
--> 769             output_retrieved=output_retrieved,
    770         )
    771 

/usr/local/lib/python3.6/dist-packages/torch/nn/modules/module.py in _call_impl(self, *input, **kwargs)
    720             result = self._slow_forward(*input, **kwargs)
    721         else:
--> 722             result = self.forward(*input, **kwargs)
    723         for hook in itertools.chain(
    724                 _global_forward_hooks.values(),

/usr/local/lib/python3.6/dist-packages/transformers/modeling_rag.py in forward(self, input_ids, attention_mask, encoder_outputs, decoder_input_ids, decoder_attention_mask, past_key_values, doc_scores, context_input_ids, context_attention_mask, use_cache, output_attentions, output_hidden_states, output_retrieved)
    589                 assert (
    590                     context_input_ids is not None
--> 591                 ), "Make sure that `context_input_ids` are passed, if no `retriever` is set. Alternatively, you can set a retriever using the `set_retriever(...)` function."
    592                 assert (
    593                     context_attention_mask is not None

AssertionError: Make sure that `context_input_ids` are passed, if no `retriever` is set. Alternatively, you can set a retriever using the `set_retriever(...)` function.
```
I suspect `context_input_ids` is not passed to `forward` method. And if model is not initialised with retriever then `forward` function complain about missing `context_input_ids` or `retriever`. Referring to following piece of code in `RagSequenceForGeneration` class and `generator` function.
```
            # then, run model forwards to get nll scores:
            new_input_ids = input_ids[index : index + 1].repeat(len(output_sequences), 1)
            outputs = self(new_input_ids, labels=output_sequences, exclude_bos_score=True)
            top_cand_inds = (-outputs["loss"]).topk(num_doc_return_sequences)[1]
```

## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
It should work as intended as `RagTokenForGeneration` do.
Hey @lalitpagaria - spot on! Thanks a lot for your issue, you're 100% correct here. 

I actually noticed that the `RagSequence` generate function is a bit more complex so that we cannot do the decomposed (embed, retrieve, generate) example here...

The PR linked to the issue removes the use case from the examples and fixes the one for `RagToken...`.<|||||>UPDATE: @patrickvonplaten  Sorry for my miss-understanding. Yes without calling generate directly fixed this with your PR. Thanks you very much for fix.

-------------------
@patrickvonplaten Thank you for update. ~~I tried your changes on my code snippets and still got same error. If you see my example I am passing `context_input_ids`~~
