removed deprecated use of Variable API from pplm example
This is same as previous [PR](https://github.com/huggingface/transformers/pull/4156) which I closed due to code styling issue. Didn't know that specific isort version was supposed to be used.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4619?src=pr&el=h1) Report
> Merging [#4619](https://codecov.io/gh/huggingface/transformers/pull/4619?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/842588c12ffbbe3502c5ab4a18646ad31d9c1e34&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/4619/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4619?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4619   +/-   ##
=======================================
  Coverage   78.02%   78.03%           
=======================================
  Files         124      124           
  Lines       20626    20626           
=======================================
+ Hits        16093    16095    +2     
+ Misses       4533     4531    -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4619?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4619/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.53% <0.00%> (+0.11%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4619/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <0.00%> (+0.16%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4619?src=pr&el=footer). Last update [842588c...8588578](https://codecov.io/gh/huggingface/transformers/pull/4619?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>@julien-c  Can you please see this ?<|||||>@sgugger Can you please see it ?