Trainer support for iterabledataset
To fix --> #5829 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5834?src=pr&el=h1) Report
> Merging [#5834](https://codecov.io/gh/huggingface/transformers/pull/5834?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d088d744adb4e5aa45262a34acab3ae9e81de169&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `40.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5834/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5834?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5834      +/-   ##
==========================================
+ Coverage   78.10%   78.11%   +0.01%     
==========================================
  Files         146      146              
  Lines       26047    26053       +6     
==========================================
+ Hits        20344    20352       +8     
+ Misses       5703     5701       -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5834?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5834/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `38.46% <40.00%> (+0.61%)` | :arrow_up: |
| [src/transformers/training\_args.py](https://codecov.io/gh/huggingface/transformers/pull/5834/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzLnB5) | `80.61% <0.00%> (+3.06%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5834?src=pr&el=footer). Last update [d088d74...edd876a](https://codecov.io/gh/huggingface/transformers/pull/5834?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>I will add a test for it.
<|||||>@sshleifer --> I have made the changes you recommended and added a test that should fail without this change. Please review and let me know if there is anything else needed.<|||||>OK Will make the changes. Also do you know where to place the dataset class to prevent failure in the tf tests?<|||||>I've made the changes. Please review and feel free to edit as needed and merge<|||||>Should be good to go. I removed the WIP. Thanks.<|||||>LGTM but will wait for review from @sgugger too