Upgrade examples to pl=0.8.1
Author: sshleifer
Pull request: yes

This upgrades to the latest PL and follows the pl docs to avoid Deprecation warnings.


### Known Issue
`trainer.test` fails in multigpu, this is not a new issue, but here we add a test that will pass when the bug is fixed upstream.


Failure discussed [here](https://github.com/PyTorchLightning/pytorch-lightning/issues/2267) can be invoked on a multigpu machine with 

```bash
pytest examples/summarization

```
This failure is not new, it also existed in previous code.
Traceback: 
```bash
examples/summarization/finetune.py:322: in main
    trainer.test(model)  # this breaks in DDP, known lightning issue. See evaluate_ch
eckpoint to recover metrics.
../.conda/envs/nb/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py:11
55: in test
    self.barrier('test_setup')
../.conda/envs/nb/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py:12
60: in barrier
    torch_distrib.barrier()
../.conda/envs/nb/lib/python3.7/site-packages/torch/distributed/distributed_c10d.py:1
484: in barrier
    _check_default_pg()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    def _check_default_pg():
        """
        Helper that checks if the default ProcessGroup has been initialized, with
        assertion

        """
        assert _default_pg is not None, \
>           "Default process group is not initialized"
E       AssertionError: Default process group is not initialized

../.conda/envs/nb/lib/python3.7/site-packages/torch/distributed/distributed_c10d.py:1
87: AssertionError
```


Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5146?src=pr&el=h1) Report
> Merging [#5146](https://codecov.io/gh/huggingface/transformers/pull/5146?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1262495a912b9cd97e2ae174fd627a9d8a502341&el=desc) will **increase** coverage by `1.66%`.
> The diff coverage is `100.00%`.

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```diff
@@            Coverage Diff             @@
##           master    #5146      +/-   ##
==========================================
+ Coverage   76.37%   78.04%   +1.66%     
==========================================
  Files         138      138              
  Lines       23772    23772              
==========================================
+ Hits        18157    18553     +396     
+ Misses       5615     5219     -396     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5146?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/5146/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `97.77% <100.00%> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5146/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.16% <0.00%> (-0.13%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5146/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.57% <0.00%> (+1.42%)` | :arrow_up: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/5146/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `78.65% <0.00%> (+2.29%)` | :arrow_up: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/5146/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `99.14% <0.00%> (+2.57%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5146/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.11% <0.00%> (+3.84%)` | :arrow_up: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/5146/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `33.43% <0.00%> (+4.77%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/5146/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90NS5weQ==) | `91.31% <0.00%> (+42.22%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5146/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `89.93% <0.00%> (+81.20%)` | :arrow_up: |

------

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Flaxy failure, merging.