change scheduler.get_last_lr to get_lr to avoid bug
Learning rate scheduler function for getting last lr is `get_lr` instead of `get_last_lr`.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3918?src=pr&el=h1) Report
> Merging [#3918](https://codecov.io/gh/huggingface/transformers/pull/3918?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/cb3c2212c79d7ff0a4a4e84c3db48371ecc1c15d&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `0.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3918/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3918?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3918      +/-   ##
==========================================
- Coverage   78.45%   78.45%   -0.01%     
==========================================
  Files         111      111              
  Lines       18521    18521              
==========================================
- Hits        14531    14530       -1     
- Misses       3990     3991       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3918?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/3918/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `43.59% <0.00%> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3918/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.59% <0.00%> (-0.17%)` | :arrow_down: |

------

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<|||||>I don’t think that’s right. Which version of PyTorch are you running?<|||||>> I don’t think that’s right. Which version of PyTorch are you running?

Well, my version is 1.2.0, I checked the API in 1.4.0, the current version is right. 
Sorry for my hurry PR,