Allow to set Adam beta1, beta2 in TrainingArgs
In some models, `beta1` and `beta2` in Adam optimizer are set to be different from the default values `(0.9, 0.999)`. For example, RoBERTa set `beta2 = 0.98`. It is thereby necessary to add `beta1` and `beta2` in `TrainingArgs` if the user wants to fine-tune RoBERTa and other similar models. Also, another hyperparameter of Adam, `adam_epsilon`, has already been added to `TrainingArgs`. For the purpose of consistency, it would be better of `adam_beta1` and `adam_beta2` are also added.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5592?src=pr&el=h1) Report
> Merging [#5592](https://codecov.io/gh/huggingface/transformers/pull/5592?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/cfbb98297449e09e5a2443b4ba76be52a71ec0f7&el=desc) will **increase** coverage by `1.21%`.
> The diff coverage is `75.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5592/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5592?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5592      +/-   ##
==========================================
+ Coverage   76.95%   78.16%   +1.21%     
==========================================
  Files         145      145              
  Lines       25317    25319       +2     
==========================================
+ Hits        19482    19790     +308     
+ Misses       5835     5529     -306     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5592?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5592/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `37.96% <0.00%> (ø)` | |
| [src/transformers/trainer\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5592/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyX3RmLnB5) | `16.53% <ø> (ø)` | |
| [src/transformers/optimization\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5592/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9vcHRpbWl6YXRpb25fdGYucHk=) | `57.65% <100.00%> (ø)` | |
| [src/transformers/training\_args.py](https://codecov.io/gh/huggingface/transformers/pull/5592/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzLnB5) | `78.00% <100.00%> (+0.44%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5592/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `43.98% <0.00%> (-49.38%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5592/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `83.20% <0.00%> (-1.76%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5592/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `79.26% <0.00%> (-0.34%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5592/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `96.77% <0.00%> (+73.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5592?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5592?src=pr&el=footer). Last update [cfbb982...9bdb1a5](https://codecov.io/gh/huggingface/transformers/pull/5592?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LGTM! Really nice!!!<|||||>I'm fine with this