Resizing embedding matrix before sending it to the optimizer.
Hi there,

This is a minor bug when fine-tuning a pre-trained language model with a larger vocabulary using the run_lm_finetuning.py script.
Nothing major but I spent a couple of hours trying to figure out why my token embeddings were not being updated with their corresponding gradient :)

This bug will rise when you add new token types in the Tokenizer.
Since the resizing was done after passing the params to the optimizer, the wrong set of params for the embedding table were optimized.

Cheers
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3532?src=pr&el=h1) Report
> Merging [#3532](https://codecov.io/gh/huggingface/transformers/pull/3532?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d38bbb225f7b847e8be4e969cb9b40e7e4d798a6&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/3532/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3532?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3532      +/-   ##
==========================================
- Coverage   77.81%   77.80%   -0.01%     
==========================================
  Files         100      100              
  Lines       17062    17062              
==========================================
- Hits        13276    13275       -1     
- Misses       3786     3787       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3532?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3532/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.81% <0.00%> (-0.14%)` | :arrow_down: |

------

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