[pl_examples] deprecate BaseTransformer.is_logger
- proc_rank is deprecated in new PL
- updates pl to 0.8.1

@williamFalcon suggestions more than welcome!


- The test that is failing is `test_bdc_multigpu`
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5140?src=pr&el=h1) Report
> Merging [#5140](https://codecov.io/gh/huggingface/transformers/pull/5140?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/84be482f6698fac822a5113735f2242c6d3abc76&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5140/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5140?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5140      +/-   ##
==========================================
- Coverage   77.19%   77.19%   -0.01%     
==========================================
  Files         133      133              
  Lines       22232    22232              
==========================================
- Hits        17162    17161       -1     
- Misses       5070     5071       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5140?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5140/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.81% <0.00%> (-0.15%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5140?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5140?src=pr&el=footer). Last update [84be482...2aad9d1](https://codecov.io/gh/huggingface/transformers/pull/5140?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
