run_pplm.py bug fix
`is_leaf` may become `False` after `.to(device=device)` function call.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4867?src=pr&el=h1) Report
> Merging [#4867](https://codecov.io/gh/huggingface/transformers/pull/4867?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/9f5d5a531d769d07403f59661884e254f8420afe&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4867/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4867?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4867   +/-   ##
=======================================
  Coverage   76.55%   76.56%           
=======================================
  Files         128      128           
  Lines       21502    21502           
=======================================
+ Hits        16461    16462    +1     
+ Misses       5041     5040    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4867?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4867/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.12% <0.00%> (-0.48%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4867/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.62% <0.00%> (+0.94%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4867?src=pr&el=footer). Last update [9f5d5a5...26de790](https://codecov.io/gh/huggingface/transformers/pull/4867?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>@songyouwei  run_pplm.py  Can you follow the Readme steps to success run it?   And  I report  this error?  Could you teach me? 

![image](https://user-images.githubusercontent.com/49581245/84686252-334a6b80-af6e-11ea-86c5-b973424ac8e7.png)

 
