Deepcopy and pickling fails for modeling_outputs
## Environment info
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- `transformers` version: 4.0.0
- Python version: 3.8
- PyTorch version (GPU?):   1.6.0
- Using GPU in script?: N/A

### Who can help
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 albert, bert, GPT2, XLM: @LysandreJik 
 tokenizers: @mfuntowicz
 Trainer: @sgugger
 Speed and Memory Benchmarks: @patrickvonplaten
 Model Cards: @julien-c
 TextGeneration: @TevenLeScao 
 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: @patrickvonplaten
 Bart: @patrickvonplaten
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 T5: @patrickvonplaten
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 examples/seq2seq: @patil-suraj
 examples/bert-loses-patience: @JetRunner
 tensorflow: @jplu
 examples/token-classification: @stefan-it
 documentation: @sgugger
 -->

## To reproduce

Steps to reproduce the behavior:

```
>>> from transformers.modeling_outputs import BaseModelOutput
>>> import torch
>>> import copy
>>> x = BaseModelOutput(last_hidden_state=torch.ones(1,))
>>> z = copy.deepcopy(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/lib/python3.8/copy.py", line 172, in deepcopy
    y = _reconstruct(x, memo, *rv)
  File "/opt/lib/python3.8/copy.py", line 263, in _reconstruct
    y = func(*args)
TypeError: __init__() missing 1 required positional argument: 'last_hidden_state'
>>> import pickle
>>> obj = pickle.dumps(x)
>>> obj_loaded = pickle.loads(obj)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() missing 1 required positional argument: 'last_hidden_state'
```

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
No failures when using deepcopy or pickle.dumps/pickle.loads
Pinging @sgugger, the king of model outputs!<|||||>That was very quick :) Thank you @sgugger and @LysandreJik !