Fix overwrite_cache behaviour for pytorch lightning examples
cc: @nateraw

Ref: #3290
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4093?src=pr&el=h1) Report
> Merging [#4093](https://codecov.io/gh/huggingface/transformers/pull/4093?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b8686174be75220d2c26a961597a39ef4921b616&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/4093/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4093?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4093   +/-   ##
=======================================
  Coverage   78.84%   78.85%           
=======================================
  Files         114      114           
  Lines       18691    18691           
=======================================
+ Hits        14737    14738    +1     
+ Misses       3954     3953    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4093?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4093/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `43.90% <0.00%> (+0.34%)` | :arrow_up: |

------

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<|||||>Not sure when it got broken, but this looks ok at first glance to me. I'll pull it down tonight as a sanity check to run it. 

Thank you! <|||||>LGTM too 😄 Thank you again, @simonepri 