Fix typos in README and bugs in RAG example code for end-to-end evaluation and finetuning
# What does this PR do?

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This PR fixes bugs in RAG example code for [end-to-end evaluation](https://github.com/huggingface/transformers/tree/master/examples/research_projects/rag#end-to-end-evaluation) and [finetuning](https://github.com/huggingface/transformers/tree/master/examples/research_projects/rag#finetuning).

## 1. Follow the file paths of reorganized examples
Also, the file paths for example code in README are updated (`example/rag/` -> `example/research_projects/rag/`)

## 2. End-to-end evaluation
```
python examples/research_projects/rag/eval_rag.py \
    --model_name_or_path facebook/rag-sequence-nq \
    --model_type rag_sequence \
    --evaluation_set path/to/dev.source \
    --gold_data_path path/to/dev.gold_data \ # parsed `biencoder-nq-dev.json` following `qa` format
    --predictions_path path/to/e2e_preds.txt \
    --eval_mode e2e \
    --gold_data_mode qa \
    --n_docs 5 \ # You can experiment with retrieving different number of documents at evaluation time
    --print_predictions \
    --recalculate
```

With the above command, I encountered a few errors:
1. an unexpected keyword argument 'clean_up_tokenization'
```
Some weights of RagSequenceForGeneration were not initialized from the model checkpoint at facebook/rag-sequence-nq and are newly initialized: ['rag.generator.lm_head.weight']
You should probably TRAIN this model on a down-stream task to be able to use it for predictions and inference.
initializing retrieval
Loading index from https://storage.googleapis.com/huggingface-nlp/datasets/wiki_dpr/
loading file https://storage.googleapis.com/huggingface-nlp/datasets/wiki_dpr/hf_bert_base.hnswSQ8_correct_phi_128.c_index.index.dpr from cache at /home/ubuntu/.cache/huggingface/transformers/a481b3aaed56325cb8901610e03e76f93b47f4284a1392d85e2ba5ce5d40d174.a382b038f1ea97c4fbad3098cd4a881a7cd4c5f73902c093e0c560511655cc0b
loading file https://storage.googleapis.com/huggingface-nlp/datasets/wiki_dpr/hf_bert_base.hnswSQ8_correct_phi_128.c_index.index_meta.dpr from cache at /home/ubuntu/.cache/huggingface/transformers/bb9560964463bc761c682818cbdb4e1662e91d25a9407afb102970f00445678c.f8cbe3240b82ffaad54506b5c13c63d26ff873d5cfabbc30eef9ad668264bab4
7it [00:00, 54.03it/s]
Traceback (most recent call last):
  File "examples/research_projects/rag/eval_rag.py", line 314, in <module>
    main(args)
  File "examples/research_projects/rag/eval_rag.py", line 300, in main
    answers = evaluate_batch_fn(args, model, questions)
  File "examples/research_projects/rag/eval_rag.py", line 134, in evaluate_batch_e2e
    print_docs=args.print_docs,
  File "/home/ubuntu/.local/share/virtualenvs/transformers-zPEj0XTF/lib/python3.7/site-packages/torch/autograd/grad_mode.py", line 26, in decorate_context
    return func(*args, **kwargs)
  File "/home/ubuntu/workspace/transformers/src/transformers/models/rag/modeling_rag.py", line 923, in generate
    **model_kwargs,
  File "/home/ubuntu/.local/share/virtualenvs/transformers-zPEj0XTF/lib/python3.7/site-packages/torch/autograd/grad_mode.py", line 26, in decorate_context
    return func(*args, **kwargs)
  File "/home/ubuntu/workspace/transformers/src/transformers/generation_utils.py", line 503, in generate
    model_kwargs = self._prepare_encoder_decoder_kwargs_for_generation(input_ids, model_kwargs)
  File "/home/ubuntu/workspace/transformers/src/transformers/generation_utils.py", line 86, in _prepare_encoder_decoder_kwargs_for_generation
    model_kwargs["encoder_outputs"]: ModelOutput = encoder(input_ids, return_dict=True, **encoder_kwargs)
  File "/home/ubuntu/.local/share/virtualenvs/transformers-zPEj0XTF/lib/python3.7/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
TypeError: forward() got an unexpected keyword argument 'clean_up_tokenization'
```

2. another unexpected keyword argument 'print_docs'
```
Some weights of RagSequenceForGeneration were not initialized from the model checkpoint at facebook/rag-sequence-nq and are newly initialized: ['rag.generator.lm_head.weight']
You should probably TRAIN this model on a down-stream task to be able to use it for predictions and inference.
initializing retrieval
Loading index from https://storage.googleapis.com/huggingface-nlp/datasets/wiki_dpr/
loading file https://storage.googleapis.com/huggingface-nlp/datasets/wiki_dpr/hf_bert_base.hnswSQ8_correct_phi_128.c_index.index.dpr from cache at /home/ubuntu/.cache/huggingface/transformers/a481b3aaed56325cb8901610e03e76f93b47f4284a1392d85e2ba5ce5d40d174.a382b038f1ea97c4fbad3098cd4a881a7cd4c5f73902c093e0c560511655cc0b
loading file https://storage.googleapis.com/huggingface-nlp/datasets/wiki_dpr/hf_bert_base.hnswSQ8_correct_phi_128.c_index.index_meta.dpr from cache at /home/ubuntu/.cache/huggingface/transformers/bb9560964463bc761c682818cbdb4e1662e91d25a9407afb102970f00445678c.f8cbe3240b82ffaad54506b5c13c63d26ff873d5cfabbc30eef9ad668264bab4
7it [00:00, 45.43it/s]
Traceback (most recent call last):
  File "examples/research_projects/rag/eval_rag.py", line 314, in <module>
    main(args)
  File "examples/research_projects/rag/eval_rag.py", line 300, in main
    answers = evaluate_batch_fn(args, model, questions)
  File "examples/research_projects/rag/eval_rag.py", line 134, in evaluate_batch_e2e
    print_docs=args.print_docs,
  File "/home/ubuntu/.local/share/virtualenvs/transformers-zPEj0XTF/lib/python3.7/site-packages/torch/autograd/grad_mode.py", line 26, in decorate_context
    return func(*args, **kwargs)
  File "/home/ubuntu/workspace/transformers/src/transformers/models/rag/modeling_rag.py", line 923, in generate
    **model_kwargs,
  File "/home/ubuntu/.local/share/virtualenvs/transformers-zPEj0XTF/lib/python3.7/site-packages/torch/autograd/grad_mode.py", line 26, in decorate_context
    return func(*args, **kwargs)
  File "/home/ubuntu/workspace/transformers/src/transformers/generation_utils.py", line 503, in generate
    model_kwargs = self._prepare_encoder_decoder_kwargs_for_generation(input_ids, model_kwargs)
  File "/home/ubuntu/workspace/transformers/src/transformers/generation_utils.py", line 86, in _prepare_encoder_decoder_kwargs_for_generation
    model_kwargs["encoder_outputs"]: ModelOutput = encoder(input_ids, return_dict=True, **encoder_kwargs)
  File "/home/ubuntu/.local/share/virtualenvs/transformers-zPEj0XTF/lib/python3.7/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
TypeError: forward() got an unexpected keyword argument 'print_docs'
```

## 3. Finetuning
```
python examples/research_projects/rag/finetune_rag.py \
    --data_dir $DATA_DIR \
    --output_dir $OUTPUT_DIR \
    --model_name_or_path $MODEL_NAME_OR_PATH \
    --model_type rag_sequence \
    --fp16 \
    --gpus 8
```
With the above command, I found two easy bugs to be fixed:
1. [missing `return parser`](https://github.com/huggingface/transformers/blob/8217d4e37fce48490a68af7e8ce902af16318132/examples/research_projects/rag/finetune_rag.py#L498) returns None to `parser` and crashes [here](https://github.com/huggingface/transformers/blob/8217d4e37fce48490a68af7e8ce902af16318132/examples/research_projects/rag/finetune_rag.py#L528-L531)
2. [duplicated argument with `num_retrieval_workers`](https://github.com/huggingface/transformers/blob/8217d4e37fce48490a68af7e8ce902af16318132/examples/research_projects/rag/finetune_rag.py#L490-L508) is also a blocker when using `finetune_rag.py`

## Environments
- Ubuntu 18.04 LTS
- Python 3.7.7
- transformers (I tried both 4.1.1 from pip and from repo https://github.com/huggingface/transformers/commit/912f6881d2b69f180522172a5283702bd8c41d9c)
- torch: 1.7.1

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.
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 Please tag fewer than 3 people.

 albert, bert, XLM: @LysandreJik
 GPT2: @LysandreJik, @patrickvonplaten
 tokenizers: @mfuntowicz
 Trainer: @sgugger
 Benchmarks: @patrickvonplaten
 Model Cards: @julien-c
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
 rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)
 Text Generation: @patrickvonplaten, @TevenLeScao
 Blenderbot, Bart, Marian, Pegasus: @patrickvonplaten
 T5: @patrickvonplaten
 Rag: @patrickvonplaten, @lhoestq
 EncoderDecoder: @patrickvonplaten
 Longformer, Reformer: @patrickvonplaten
 TransfoXL, XLNet: @TevenLeScao, @patrickvonplaten
 examples/seq2seq: @patil-suraj
 examples/bert-loses-patience: @JetRunner
 tensorflow: @jplu
 examples/token-classification: @stefan-it
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@patrickvonplaten @lhoestq

Hi @patrickvonplaten,

Thank you for reviewing this PR!
As commented above, the argument `num_retrieval_workers` in `add_ray_specific_args` is duplicate ([first defined in `add_retriever_specific_args`](https://github.com/huggingface/transformers/blob/8217d4e37fce48490a68af7e8ce902af16318132/examples/research_projects/rag/finetune_rag.py#L490-L508)) and causes an error.<|||||>Great work @yoshitomo-matsubara <|||||>Thank you for reviewing PR @patrickvonplaten @lhoestq !