Make PyTorch model files independent from each other
As discussed after the survey and expressed in the project README, our goal is to have independent model files even if it means some code is duplicated. This PR fixes this for all PyTorch models except:
- the full subclasses (CamemBERT, FlauBERT, XLM-RoBERTa),
- the BART-like models (BART, mBART, marian, Pegasus)
- the "composite" models (BertGeneration, DPR and RetriBERT).

The first ones should stay as is, as we discussed internally, the second ones will be dealt with in another PR and I personally think the last ones (which directly import `BertModel`) should also stay as is.

This leverages the script introduced in #7219 to make sure the identical copies stay True to the original.

Also, as discussed with Lysandre, I removed the XxxLayerNorm when it was just `nn.LayerNorm`.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7352?src=pr&el=h1) Report
> Merging [#7352](https://codecov.io/gh/huggingface/transformers/pull/7352?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/129fdae04033fe4adfe013b734deaec6ec34ae2e?el=desc) will **increase** coverage by `1.25%`.
> The diff coverage is `71.95%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7352/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7352?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7352      +/-   ##
==========================================
+ Coverage   76.68%   77.93%   +1.25%     
==========================================
  Files         181      181              
  Lines       34851    35140     +289     
==========================================
+ Hits        26724    27385     +661     
+ Misses       8127     7755     -372     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7352?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/7352/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `18.69% <22.72%> (-52.92%)` | :arrow_down: |
| [src/transformers/activations.py](https://codecov.io/gh/huggingface/transformers/pull/7352/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9ucy5weQ==) | `79.16% <50.00%> (-2.66%)` | :arrow_down: |
| [src/transformers/modeling\_retribert.py](https://codecov.io/gh/huggingface/transformers/pull/7352/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yZXRyaWJlcnQucHk=) | `34.24% <50.00%> (ø)` | |
| [src/transformers/modeling\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/7352/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tb2JpbGViZXJ0LnB5) | `89.38% <90.00%> (-0.08%)` | :arrow_down: |
| [src/transformers/modeling\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/7352/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbGVjdHJhLnB5) | `84.98% <90.95%> (+2.79%)` | :arrow_up: |
| [src/transformers/modeling\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/7352/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hbGJlcnQucHk=) | `84.16% <97.14%> (+0.69%)` | :arrow_up: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/7352/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `88.36% <100.00%> (+0.08%)` | :arrow_up: |
| [src/transformers/modeling\_funnel.py](https://codecov.io/gh/huggingface/transformers/pull/7352/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19mdW5uZWwucHk=) | `86.74% <100.00%> (-0.02%)` | :arrow_down: |
| [src/transformers/modeling\_layoutlm.py](https://codecov.io/gh/huggingface/transformers/pull/7352/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sYXlvdXRsbS5weQ==) | `94.45% <100.00%> (ø)` | |
| [src/transformers/modeling\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/7352/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19seG1lcnQucHk=) | `90.73% <100.00%> (-0.04%)` | :arrow_down: |
| ... and [15 more](https://codecov.io/gh/huggingface/transformers/pull/7352/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/7352?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/7352?src=pr&el=footer). Last update [129fdae...90a918f](https://codecov.io/gh/huggingface/transformers/pull/7352?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
