Unable to run PPLM potato example
I am following the instructions mentioned in the below page to run PPLM examples. Unfortunately the code is throwing error. 

PPLM Page : https://github.com/huggingface/transformers/tree/master/examples/research_projects/pplm


**Environment:**
Python 3.9.7
pytorch-lightning==1.0.4


Error Log:
```
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
= Prefix of sentence =
<|endoftext|>The potato

Using PPLM-BoW
  0%|                                                                                                                                                                                 | 0/50 [00:00<?, ?it/s<|endoftext|>The potato has
  2%|###3                                                                                                                                                                     | 1/50 [00:00<00:29,  1.63it/s]<|endoftext|>The potato has been
  4%|######7                                                                                                                                                                  | 2/50 [00:00<00:21,  2.28it/s]<|endoftext|>The potato has been in
  6%|##########1                                                                                                                                                              | 3/50 [00:01<00:19,  2.39it/s]<|endoftext|>The potato has been in the
  8%|#############5                                                                                                                                                           | 4/50 [00:01<00:16,  2.73it/s]<|endoftext|>The potato has been in the news
 10%|################9                                                                                                                                                        | 5/50 [00:01<00:15,  2.89it/s]<|endoftext|>The potato has been in the news lately
 12%|####################2                                                                                                                                                    | 6/50 [00:02<00:14,  2.99it/s]<|endoftext|>The potato has been in the news lately for
 14%|#######################6                                                                                                                                                 | 7/50 [00:02<00:14,  3.01it/s]<|endoftext|>The potato has been in the news lately for its
 16%|###########################                                                                                                                                              | 8/50 [00:02<00:13,  3.01it/s]<|endoftext|>The potato has been in the news lately for its alleged
 18%|##############################4                                                                                                                                          | 9/50 [00:03<00:13,  3.01it/s]<|endoftext|>The potato has been in the news lately for its alleged role
 20%|#################################6                                                                                                                                      | 10/50 [00:03<00:13,  3.00it/s]<|endoftext|>The potato has been in the news lately for its alleged role in
 22%|####################################9                                                                                                                                   | 11/50 [00:03<00:13,  2.89it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a
 24%|########################################3                                                                                                                               | 12/50 [00:04<00:13,  2.83it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly
 26%|###########################################6                                                                                                                            | 13/50 [00:04<00:13,  2.79it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague
 28%|###############################################                                                                                                                         | 14/50 [00:05<00:13,  2.76it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that
 30%|##################################################4                                                                                                                     | 15/50 [00:05<00:12,  2.72it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed
 32%|#####################################################7                                                                                                                  | 16/50 [00:05<00:12,  2.70it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1
 34%|#########################################################1                                                                                                              | 17/50 [00:06<00:12,  2.64it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,
 36%|############################################################4                                                                                                           | 18/50 [00:06<00:12,  2.57it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400
 38%|###############################################################8                                                                                                        | 19/50 [00:07<00:12,  2.53it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people
 40%|###################################################################2                                                                                                    | 20/50 [00:07<00:11,  2.50it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across
 42%|######################################################################5                                                                                                 | 21/50 [00:07<00:11,  2.47it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe
 44%|#########################################################################9                                                                                              | 22/50 [00:08<00:11,  2.44it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe.
 46%|#############################################################################2                                                                                          | 23/50 [00:08<00:11,  2.36it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It
 48%|################################################################################6                                                                                       | 24/50 [00:09<00:11,  2.31it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's
 50%|####################################################################################                                                                                    | 25/50 [00:09<00:11,  2.25it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a
 52%|#######################################################################################3                                                                                | 26/50 [00:10<00:10,  2.23it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato
 54%|##########################################################################################7                                                                             | 27/50 [00:10<00:10,  2.21it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with
 56%|##############################################################################################                                                                          | 28/50 [00:11<00:10,  2.19it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an
 58%|#################################################################################################4                                                                      | 29/50 [00:11<00:09,  2.14it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate
 60%|####################################################################################################8                                                                   | 30/50 [00:12<00:09,  2.08it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history
 62%|########################################################################################################1                                                               | 31/50 [00:12<00:10,  1.83it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.
 64%|###########################################################################################################5                                                            | 32/50 [00:13<00:10,  1.73it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

 66%|##############################################################################################################8                                                         | 33/50 [00:13<00:09,  1.79it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.


 68%|##################################################################################################################2                                                     | 34/50 [00:14<00:09,  1.72it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In
 70%|#####################################################################################################################6                                                  | 35/50 [00:15<00:09,  1.61it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact
 72%|########################################################################################################################9                                               | 36/50 [00:15<00:08,  1.66it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact,
 74%|############################################################################################################################3                                           | 37/50 [00:16<00:08,  1.54it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes
 76%|###############################################################################################################################6                                        | 38/50 [00:17<00:07,  1.62it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes were
 78%|###################################################################################################################################                                     | 39/50 [00:17<00:06,  1.66it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes were actually
 80%|######################################################################################################################################4                                 | 40/50 [00:19<00:08,  1.21it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes were actually bred
 82%|#########################################################################################################################################7                              | 41/50 [00:19<00:07,  1.18it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes were actually bred to
 84%|#############################################################################################################################################1                          | 42/50 [00:21<00:07,  1.09it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes were actually bred to kill
 86%|################################################################################################################################################4                       | 43/50 [00:22<00:06,  1.01it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes were actually bred to kill people
 88%|###################################################################################################################################################8                    | 44/50 [00:22<00:05,  1.08it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes were actually bred to kill people.
 90%|#######################################################################################################################################################2                | 45/50 [00:23<00:04,  1.06it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes were actually bred to kill people. But
 92%|##########################################################################################################################################################5             | 46/50 [00:24<00:03,  1.12it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes were actually bred to kill people. But in
 94%|#############################################################################################################################################################9          | 47/50 [00:25<00:02,  1.09it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes were actually bred to kill people. But in the
 96%|#################################################################################################################################################################2      | 48/50 [00:26<00:01,  1.19it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes were actually bred to kill people. But in the early
 98%|####################################################################################################################################################################6   | 49/50 [00:27<00:00,  1.17it/s]<|endoftext|>The potato has been in the news lately for its alleged role in a deadly plague that killed 1,400 people across Europe. It's a potato with an unfortunate history.

In fact, potatoes were actually bred to kill people. But in the early 1800
100%|########################################################################################################################################################################| 50/50 [00:27<00:00,  1.79it/s]
  0%|                                                                                                                                                                                 | 0/50 [00:00<?, ?it/s]
Traceback (most recent call last):
  File "/Users/apple/PycharmProjects/streamlit-pplm/transformers/examples/research_projects/pplm/run_pplm.py", line 827, in <module>
    run_pplm_example(**vars(args))
  File "/Users/apple/PycharmProjects/streamlit-pplm/transformers/examples/research_projects/pplm/run_pplm.py", line 663, in run_pplm_example
    unpert_gen_tok_text, pert_gen_tok_texts, _, _ = full_text_generation(
  File "/Users/apple/PycharmProjects/streamlit-pplm/transformers/examples/research_projects/pplm/run_pplm.py", line 389, in full_text_generation
    pert_gen_tok_text, discrim_loss, loss_in_time = generate_text_pplm(
  File "/Users/apple/PycharmProjects/streamlit-pplm/transformers/examples/research_projects/pplm/run_pplm.py", line 499, in generate_text_pplm
    pert_past, _, grad_norms, loss_this_iter = perturb_past(
  File "/Users/apple/PycharmProjects/streamlit-pplm/transformers/examples/research_projects/pplm/run_pplm.py", line 115, in perturb_past
    grad_accumulator = [(np.zeros(p.shape).astype("float32")) for p in past]
  File "/Users/apple/PycharmProjects/streamlit-pplm/transformers/examples/research_projects/pplm/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'

```
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