Seq2SeqDataset uses linecache to save memory by @Pradhy729 (#5792) 
The isort check doesn't fail when I run it on my end. Other than that - I would say this is complete.
<|||||>Awesome, I'm testing it now on WMT english-romanian translation.<|||||>OK, I have it running with many modifications and it works much better. Very little cpu ram wasted!

Is it OK if I make a new PR or do you prefer to add me as a contributor to your fork and so I can push to this branch? either way you will get all the credit in the release notes/pr summary :) 

<|||||>Thanks - I'll add you as a contributor in mine. :)<|||||>> Thanks - I'll add you as a contributor in mine. :)

You can just click on "Allow edits from maintainers" on your PR. (in case you didn't know this feature)<|||||>Any updates here? Is it good to go?
<|||||>I'm still working on cleaning up my code. Sorry for the delay. <|||||>Biggest change:
- For MBart Tokenizer, we can't use the `encode_line` approach because there are special tokens all over the place, so I made a separate dataset. 

Stylistic:
- Address the `linecache` off by 1 error inside of `__getitem__` instead other places.
- `_get_examples` -> `get_char_lens`.
- `MbartTokenizer` cleanup.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5792?src=pr&el=h1) Report
> Merging [#5792](https://codecov.io/gh/huggingface/transformers/pull/5792?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/eae6d8d14f1d25d62c3fe9e7e410607bbaf69787&el=desc) will **increase** coverage by `0.91%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5792/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5792?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5792      +/-   ##
==========================================
+ Coverage   77.54%   78.46%   +0.91%     
==========================================
  Files         146      146              
  Lines       26200    26200              
==========================================
+ Hits        20318    20559     +241     
+ Misses       5882     5641     -241     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5792?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/5792/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `95.45% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5792/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `26.02% <0.00%> (-69.52%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5792/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.21% <0.00%> (-1.26%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5792/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `96.77% <0.00%> (+73.38%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5792?src=pr&el=footer). Last update [eae6d8d...79d73ee](https://codecov.io/gh/huggingface/transformers/pull/5792?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
