GPT2 error when we try to run torch.jit.script 
## Environment info
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- `transformers` version: 4.9.2
- Platform: Ubuntu 18.04
- Python version: Python3.6
- PyTorch version (GPU?): 1.9.0 GPU
- Tensorflow version (GPU?): N/A
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help 
@patrickvonplaten, @LysandreJik

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Models:

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## Information

Model I am using (Bert, XLNet ...): GPT2

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

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

## To reproduce
I was trying to run torch.jit.script and I get the following error from JIT frontend 

```
 File "/home/ubuntu/anaconda3/envs/python3/lib/python3.6/site-packages/torch/jit/frontend.py", line 330, in __call__
    raise UnsupportedNodeError(ctx, node)
torch.jit.frontend.UnsupportedNodeError: GeneratorExp aren't supported:
  File "/home/ubuntu/anaconda3/envs/python3/lib/python3.6/site-packages/transformers/models/gpt2/modeling_gpt2.py", line 756
                # Ensure layer_past is on same device as hidden_states (might not be correct)
                if layer_past is not None:
                    layer_past = tuple(past_state.to(hidden_states.device) for past_state in layer_past)
                                       ~ <--- HERE
                # Ensure that attention_mask is always on the same device as hidden_states
                if attention_mask is not None:
```

I am curious to know if this is a known issue or learn if I am doing something wrong.
#### Sample Code:

```
from transformers import GPT2LMHeadModel, GPT2Config
import torch
configuration = GPT2Config(n_embd=1600, n_layer=48, n_head=25)
model = GPT2LMHeadModel(configuration)
script_model = torch.jit.script(model.base_model)
```

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

Can you try the following: 

```python
from transformers import GPT2LMHeadModel, GPT2Config
import torch
configuration = GPT2Config(n_embd=1600, n_layer=48, n_head=25, use_cache=False)
model = GPT2LMHeadModel(configuration)
script_model = torch.jit.script(model.base_model)
```

just to first see whether disabling the cache solves the problem<|||||>> Can you try the following:
> 
> ```python
> from transformers import GPT2LMHeadModel, GPT2Config
> import torch
> configuration = GPT2Config(n_embd=1600, n_layer=48, n_head=25, use_cache=False)
> model = GPT2LMHeadModel(configuration)
> script_model = torch.jit.script(model.base_model)
> ```
> 
> just to first see whether disabling the cache solves the problem

I still see the same error.
```
torch.jit.frontend.UnsupportedNodeError: GeneratorExp aren't supported:
  File "/home/ubuntu/anaconda3/envs/python3/lib/python3.6/site-packages/transformers/models/gpt2/modeling_gpt2.py", line 756
                # Ensure layer_past is on same device as hidden_states (might not be correct)
                if layer_past is not None:
                    layer_past = tuple(past_state.to(hidden_states.device) for past_state in layer_past)
                                       ~ <--- HERE
                # Ensure that attention_mask is always on the same device as hidden_states
                if attention_mask is not None:
```

This seems like GPT model as it is implemented today is not supported by `torch.jit.script` because the model uses generator expr and Script doesn't support it yet. Is this accurate or am I missing anything? Also, would `torch.jit.trace`ing the GPT2 model cause any correctness issues? <|||||>Hello! The documentation regarding TorchScript can be found [here](https://huggingface.co/transformers/serialization.html#torchscript).

You're correct in your analysis: `torch.jit.script` isn't supported, while `torch.jit.trace` is supported!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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