PPLM example - AttributeError issue
Hi all,
Thank you for great library.
Now I try to understand PPLM model (see reference https://eng.uber.com/pplm/ ), and when I try to start example from HuggingFace repository https://github.com/huggingface/transformers/tree/master/examples/research_projects/pplm (run_pplm.py) - I faced with the next issue:


```
Traceback (most recent call last):
  File "run_pplm.py", line 820, in <module>
    run_pplm_example(**vars(args))
  File "run_pplm.py", line 678, in run_pplm_example
    repetition_penalty=repetition_penalty,
  File "run_pplm.py", line 405, in full_text_generation
    repetition_penalty=repetition_penalty,
  File "run_pplm.py", line 511, in generate_text_pplm
    device=device,
  File "run_pplm.py", line 115, in perturb_past
    grad_accumulator = [(np.zeros(p.shape).astype("float32")) for p in past]
  File "run_pplm.py", line 115, in <listcomp>
    grad_accumulator = [(np.zeros(p.shape).astype("float32")) for p in past]
AttributeError: 'tuple' object has no attribute 'shape'

```
## 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:
- Platform: 4.3.0.dev0
- Python version: 3.8
- PyTorch version (GPU?): 1.7.1

I started this example on my own laptop and in Google Colab environment

## To reproduce

Steps to reproduce the behavior:

1. Follow to https://github.com/huggingface/transformers/tree/master/examples/research_projects/pplm and do Setup steps
2. Do command: `python run_pplm.py -B military --cond_text "The potato" --length 50 --gamma 1.5 --num_iterations 3 --num_samples 10 --stepsize 0.03 --window_length 5 --kl_scale 0.01 --gm_scale 0.99 --colorama --sample
`

## Expected behavior
This script should work without an error :)

Hi! Could you install an earlier version of `transformers` to see if it works? I believe it was tested with `transformers==3.0.1`<|||||>It works fine up to and including version v4.2.2 but is broken in versions above that<|||||>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.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.