Fix inconsistency between T5WithLMHeadModel's doc and it's behavior
The doc string for `T5WithLMHeadModel` is currently inconsistent with it's behavior.

The doc string says that the forward method ignores indices of -1 when computing the loss; however, the method instead ignores indices of -100. This pull request changes the method to ignore indices of -1, making the two consistent.

It's worth noting, there is a [commit](https://github.com/huggingface/transformers/commit/1b59b57b57010e6119282f3dbf37f8c7c6d6313e#diff-7370db3a19209bf984cc40925aaf2b71) by @thomwolf that changed this value from -1 to -100, though I couldn't find why the change was made. Thomas, perhaps you remember if the change is still important? If it is, I can instead update this PR to change the doc string.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2607?src=pr&el=h1) Report
> Merging [#2607](https://codecov.io/gh/huggingface/transformers/pull/2607?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1a8e87be4e2a1b551175bd6f0f749f3d2289010f?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

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```diff
@@           Coverage Diff           @@
##           master    #2607   +/-   ##
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  Coverage   74.53%   74.53%           
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  Files          87       87           
  Lines       14819    14819           
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  Hits        11046    11046           
  Misses       3773     3773
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2607?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/2607/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `81.09% <ø> (ø)` | :arrow_up: |

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<|||||>Hi! Indeed, we changed all the Cross Entropy ignored indices to be -100 to respect the PyTorch default. The docstrings would need to be changed, instead of the index. We could even remove the argument:

```py
loss_fct = CrossEntropyLoss()
```

to keep in sync with pytorch.<|||||>@LysandreJik, that makes perfect sense. Thanks for the background!

I've updated this PR to change the doc string instead. 

Removing the `ignore_index` argument would mean that the behavior of the method depends on which PyTorch version a user has installed, which could be counter-intuitive. Similarly, to ensure the doc string is accurate we'd have to add a unit test for it and run against all versions of PyTorch that a user might reasonably install.

For simplicity / consistency, I'd suggest continuing to explicitly pass the argument.

I'm happy to add a commit removing it though, if you feel otherwise.<|||||>Thanks for updating, we can keep the `ignore_index` argument.

Thanks @nalourie-ai2 !