Custom TF weights loading
Author: jplu
Pull request: yes

This PR provides a custom weight loading function in order to take into account dynamic model architecture building. More precisely, the brand new loading function takes into account the `authorized_unexpected_keys` and `authorized_missing_keys` class attributes enabling the possibility to ignore some layers in the models.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7422?src=pr&el=h1) Report
> Merging [#7422](https://codecov.io/gh/huggingface/transformers/pull/7422?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/95f792afb0f0ce5a7b4f0e8df108b10157a69134?el=desc) will **decrease** coverage by `0.21%`.
> The diff coverage is `87.50%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7422/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7422?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7422      +/-   ##
==========================================
- Coverage   78.51%   78.30%   -0.22%     
==========================================
  Files         184      181       -3     
  Lines       36734    35917     -817     
==========================================
- Hits        28843    28125     -718     
+ Misses       7891     7792      -99     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7422?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `88.37% <50.00%> (-1.94%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.06% <92.30%> (+0.84%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/7422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `98.91% <100.00%> (+<0.01%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/7422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `24.59% <0.00%> (-72.35%)` | :arrow_down: |
| [src/transformers/configuration\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/7422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX21vYmlsZWJlcnQucHk=) | `26.47% <0.00%> (-70.59%)` | :arrow_down: |
| [src/transformers/modeling\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/7422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tb2JpbGViZXJ0LnB5) | `23.51% <0.00%> (-65.93%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/7422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `58.52% <0.00%> (-34.74%)` | :arrow_down: |
| [src/transformers/modeling\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/7422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19seG1lcnQucHk=) | `69.91% <0.00%> (-20.82%)` | :arrow_down: |
| [src/transformers/trainer\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyX3V0aWxzLnB5) | `63.30% <0.00%> (-5.35%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/7422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `63.23% <0.00%> (-1.59%)` | :arrow_down: |
| ... and [24 more](https://codecov.io/gh/huggingface/transformers/pull/7422/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7422?src=pr&el=footer). Last update [95f792a...6f52cc9](https://codecov.io/gh/huggingface/transformers/pull/7422?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Just merged your suggestions :)

@patrickvonplaten I have done some updates, let me know if it looks like what you have in mind.

There is an issue with Longformer apparently.

Ok, I found why, and I should have thought about this much before.... 😣 

We cannot have `None` into a tuple, the logic works only when the `return_dict` is True.

@LysandreJik are we able to merge?

Good to merge for me

Ran the slow tests, they pass.