consistent ignore keys + make private
This PR addresses https://github.com/huggingface/transformers/issues/7258

(the proposal has evolved a bit since the initial PR, this comments reflects the current state)

* [x] renames optional model attributes:
```
  - authorized_missing_keys    => _keys_to_ignore_on_load_missing
  - authorized_unexpected_keys => _keys_to_ignore_on_load_unexpected
  - keys_to_never_save         => _keys_to_ignore_on_save
```
to (1) make them consistent (2) make them private

* [x] removes these from public API docstring (documents them privately as comments in place)

This is a breaking change.

Fixes https://github.com/huggingface/transformers/issues/7258

@LysandreJik, @sgugger 

p.s. if we want to postpone it for v5, this PR was a quick one liner:
```
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's|authorized_missing_keys|_keys_to_ignore_on_load_missing|g; s|authorized_unexpected_keys|_keys_to_ignore_on_load_unexpected|g'; s|keys_to_never_save|_keys_to_ignore_on_save|g' {} \;
```
and then manually adjusting the docs.
oh, boy, there is also `authorized_unexpected_keys`

https://github.com/huggingface/transformers/blob/49759c0cda29ab614b81e0869972c99f2edba7aa/src/transformers/modeling_tf_utils.py#L346-L354
<|||||>Indeed, very nice catch! How should we rename that one? <|||||>Current `authorized_missing_keys` and `authorized_unexpected_keys` do the same thing overall, just 2 different categories.

perhaps?

```
  - authorized_missing_keys    => _keys_to_ignore_on_load_missing
  - authorized_unexpected_keys => _keys_to_ignore_on_load_unexpected
  - keys_to_never_save         => _keys_to_ignore_on_save
```<|||||>For me `authorized_unexpected_keys` should be the `_keys_to_ignore_on_load`: they are in the state dict but we ignore them.
The `authorized_missing_keys` should have another name such as `_keys_missing_to_ignore_on_load` or just `_keys_missing_to_ignore`.

Re- documentation. We usually document private stuff in comments in the code, so I think we should remove the public documentation and change it in comments.<|||||>We were writing at the same time @stas00 , your names are better than mine. Go ahead!<|||||>You can safely ignore the failed connections. It's been happening since the change to git-based repos. We're looking into fixing it with @julien-c, it happens very often.<|||||>@LysandreJik, I trust you will document this breaking change - I just don't know where I'd do that...<|||||>Yes, I'm currently documenting all breaking changes in the release notes.