[pl_examples] revert deletion of optimizer_step
using default `optimizer_step` has at least 2 issues:
The default version...
1) doesn't call `lr_scheduler.step()`
2) does call `self.trainer.scaler.step(optimizer)`

I haven't diagnosed which of these is the main culprit of the issue I was seeing (very high loss, not going down). This fixes that issue. 

I suspect it is mostly the latter: @williamFalcon 

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5227?src=pr&el=h1) Report
> Merging [#5227](https://codecov.io/gh/huggingface/transformers/pull/5227?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c01480bba3b2f0bd8516679476235f4701c21b3b&el=desc) will **increase** coverage by `0.06%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5227/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5227?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5227      +/-   ##
==========================================
+ Coverage   77.98%   78.05%   +0.06%     
==========================================
  Files         138      138              
  Lines       23839    23839              
==========================================
+ Hits        18592    18608      +16     
+ Misses       5247     5231      -16     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5227?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5227/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `76.42% <0.00%> (-0.39%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5227/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.86% <0.00%> (-0.30%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5227/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.33% <0.00%> (-0.24%)` | :arrow_down: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/5227/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `35.03% <0.00%> (+6.36%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5227?src=pr&el=footer). Last update [c01480b...51bb9b3](https://codecov.io/gh/huggingface/transformers/pull/5227?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
