revert erroneous fix #2276
I based #2276 on having an error pop up on an older pytorch version, and also on the erroneous (current!) documentation for pytorch.Tensor.scatter():

> `scatter(dim, index, source)` → Tensor
> 
>     Out-of-place version of torch.Tensor.scatter_()
> 
> `scatter_(dim, index, src)` → Tensor
> ...

The argument was called `source`, inconsistently, in the version I was using, but somewhere along the way it went back to being `src` without the docs changing, which caused this confusion...
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2310?src=pr&el=h1) Report
> Merging [#2310](https://codecov.io/gh/huggingface/transformers/pull/2310?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/81db12c3ba0c2067f43c4a63edf5e45f54161042?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2310/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2310?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2310   +/-   ##
=======================================
  Coverage   73.54%   73.54%           
=======================================
  Files          87       87           
  Lines       14789    14789           
=======================================
  Hits        10876    10876           
  Misses       3913     3913
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2310?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2310/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `63.91% <0%> (ø)` | :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/2310?src=pr&el=footer). Last update [81db12c...e1844d9](https://codecov.io/gh/huggingface/transformers/pull/2310?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Hi, probably best to just use positional arguments here (instead of keywords) then, don't you think?<|||||>Great, thanks!