RagTokenForGeneration.from_pretrained fails while running demo script
## Environment info
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- `transformers` version: 3.3.1
- Platform: Linux-4.4.0-1113-aws-x86_64-with-debian-stretch-sid
- Python version: 3.7.9
- PyTorch version (GPU?): 1.3.1 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no

### Who can help
@VictorSanh 
@patrickvonplaten 
@sshleifer
transformers/modeling_utils.py
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 TextGeneration: @TevenLeScao 
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 nlp datasets: [different repo](https://github.com/huggingface/nlp)
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## Information

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

The problem arises when using:
* [x] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [x] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. install new conda env py=3.7
2. install RAG requirements
3. run example code from https://huggingface.co/transformers/master/model_doc/rag.html
```python
Python 3.7.9 (default, Aug 31 2020, 12:42:55)
[GCC 7.3.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from transformers import RagTokenizer, RagRetriever, RagTokenForGeneration
>>> import torch
>>> tokenizer = RagTokenizer.from_pretrained("facebook/rag-token-nq")
>>> retriever = RagRetriever.from_pretrained("facebook/rag-token-nq", index_name="exact", use_dummy_dataset=True)
Using custom data configuration dummy.psgs_w100.nq.no_index
Reusing dataset wiki_dpr (/homes/thielk/.cache/huggingface/datasets/wiki_dpr/dummy.psgs_w100.nq.no_index/0.0.0/14b973bf2a456087ff69c0fd34526684eed22e48e0dfce4338f9a22b965ce7c2)
Using custom data configuration dummy.psgs_w100.nq.exact
Reusing dataset wiki_dpr (/homes/thielk/.cache/huggingface/datasets/wiki_dpr/dummy.psgs_w100.nq.exact/0.0.0/14b973bf2a456087ff69c0fd34526684eed22e48e0dfce4338f9a22b965ce7c2)
>>> model = RagTokenForGeneration.from_pretrained("facebook/rag-token-nq", retriever=retriever)
```

stack trace:

```python
Traceback (most recent call last):
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/tarfile.py", line 187, in nti
    n = int(s.strip() or "0", 8)
ValueError: invalid literal for int() with base 8: 'del.embe'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/tarfile.py", line 2289, in next
    tarinfo = self.tarinfo.fromtarfile(self)
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/tarfile.py", line 1095, in fromtarfile
    obj = cls.frombuf(buf, tarfile.encoding, tarfile.errors)
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/tarfile.py", line 1037, in frombuf
    chksum = nti(buf[148:156])
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/tarfile.py", line 189, in nti
    raise InvalidHeaderError("invalid header")
tarfile.InvalidHeaderError: invalid header

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/site-packages/torch/serialization.py", line 595, in _load
    return legacy_load(f)
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/site-packages/torch/serialization.py", line 506, in legacy_load
    with closing(tarfile.open(fileobj=f, mode='r:', format=tarfile.PAX_FORMAT)) as tar, \
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/tarfile.py", line 1593, in open
    return func(name, filemode, fileobj, **kwargs)
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/tarfile.py", line 1623, in taropen
    return cls(name, mode, fileobj, **kwargs)
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/tarfile.py", line 1486, in __init__
    self.firstmember = self.next()
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/tarfile.py", line 2301, in next
    raise ReadError(str(e))
tarfile.ReadError: invalid header

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/site-packages/transformers/modeling_utils.py", line 927, in from_pretrained
    state_dict = torch.load(resolved_archive_file, map_location="cpu")
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/site-packages/torch/serialization.py", line 426, in load
    return _load(f, map_location, pickle_module, **pickle_load_args)
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/site-packages/torch/serialization.py", line 599, in _load
    raise RuntimeError("{} is a zip archive (did you mean to use torch.jit.load()?)".format(f.name))
RuntimeError: /homes/thielk/.cache/torch/transformers/06fe449ffe41cbe16aeb1f5976989313464a3c44a605e9a8b91bf6440dfa6026.696574d8c17eafbac08f43f01e951252057f8feb133b64a33b76d4c47d65367a is a zip archive (did you mean to use torch.jit.load()?)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/homes/thielk/miniconda3/envs/transformers-pytorch/lib/python3.7/site-packages/transformers/modeling_utils.py", line 930, in from_pretrained
    "Unable to load weights from pytorch checkpoint file. "
OSError: Unable to load weights from pytorch checkpoint file. If you tried to load a PyTorch model from a TF 2.0 checkpoint, please set from_tf=True.
```
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## Expected behavior
be able to completely run example code from RAG documentation
May be related to #7583 
<!-- A clear and concise description of what you would expect to happen. -->
```python
from transformers import RagTokenizer, RagRetriever, RagTokenForGeneration
import torch
tokenizer = RagTokenizer.from_pretrained("facebook/rag-token-nq")
retriever = RagRetriever.from_pretrained("facebook/rag-token-nq", index_name="exact", use_dummy_dataset=True)
# initialize with RagRetriever to do everything in one forward call
model = RagTokenForGeneration.from_pretrained("facebook/rag-token-nq", retriever=retriever)
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"]
outputs = model(input_ids=input_ids, labels=input_dict["labels"])
# or use retriever seperately
model = RagTokenForGeneration.from_pretrained("facebook/rag-token-nq", use_dummy_dataset=True)
# 1. Encode
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")
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(context_input_ids=docs_dict["context_input_ids"], context_attention_mask=docs_dict["context_attention_mask"], doc_scores=doc_scores, decoder_input_ids=input_dict["labels"])
# or directly generate
generated = model.generate(input_ids=input_dict["input_ids"])
generated_string = tokenizer.batch_decode(generated, skip_special_tokens=True)
```
This may be related to an incompatible pytorch or cuda version<|||||>@mthielk - this can very well be due to the PyTorch version. Did you try with a more current version of PyTorch? <|||||>@patrickvonplaten I face the same issue with PyTorch version 1.4.0.  <|||||>I can confirm that this error occurs with PyTorch version 1.4.0!<|||||>Okey, after some internal discussion the error is the following. PyTorch changed its `torch.save()` method officially in PyTorch 1.6.0 (check https://github.com/pytorch/pytorch/releases for 1.6.0 under "Deprecations") which means that models saved with torch >= 1.6.0 are not loadable with torch <= 1.4.0 -> hence this error. So for RAG the minimum required torch version is torch 1.5.0 it seems. (thanks @sgugger @LysandreJik )