Fix label renaming and add a battery of tests

This PR makes some changes to label renaming in `to_tf_dataset()`, both to fix some issues when users input something we weren't expecting, and also to make it easier to deprecate label renaming in future, if/when we want to move this special-casing logic to a function in `transformers`.

The main changes are:
- Label renaming now only happens when the `auto_rename_labels` argument is set. For backward compatibility, this defaults to `True` for now.
- If the user requests "label" but the data collator renames that column to "labels", the label renaming logic will now handle that case correctly.
- Added a battery of tests to make this more reliable in future.
- Adds an optimization to loading in `to_tf_dataset()` for unshuffled datasets (uses slicing instead of a list of indices)

Fixes #4772

Comments:
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Why don't we deprecate label renaming already instead ?

I think it'll break a lot of workflows if we deprecate it now! There isn't really a non-deprecated workflow yet - once we've added the `auto_rename_labels` option, then we can have `prepare_tf_dataset` on the `transformers` side use that, and then we can consider setting the default option to `False`, or beginning to deprecate it somehow.

I'm worried it's a bit of a waste of time to continue working on this behavior that shouldn't be here in the first place. Do you have a plan in mind ?

@lhoestq Broadly! The plan is:

1) Create the `auto_rename_labels` flag with this PR and skip label renaming if it isn't set. Leave it as `True` for backward compatibility.
2) Add the label renaming logic to `model.prepare_tf_dataset` in `transformers`. That method calls `to_tf_dataset()` right now. Once the label renaming logic is moved there, `model.prepare_tf_dataset` will set `auto_rename_labels=False` when calling `to_tf_dataset()`, and do label renaming itself.

After step 2, `auto_rename_labels` is now only necessary for backward compatibility when users use `to_tf_dataset` directly. I want to leave it alone for a while because the `model.prepare_tf_dataset` workflow is very new. However, once it is established, we can deprecate `auto_rename_labels` and then finally remove it from the `datasets` code and keep it in `transformers` where it belongs.

I see ! Could it be possible to not add `auto_rename_labels` at all, since you want to remove it at the end ? Something roughly like this:
1. show a warning in `to_tf_dataset` whevener a label is renamed automatically, saying that in the next major release this will be removed
1. add the label renaming logic in `transformers` (to not have the warning)
1. after some time, do a major release 3.0.0 and remove label renaming completely in `to_tf_dataset`

What do you think ? cc @LysandreJik in case you have an opinion on this process.

@lhoestq I think that plan is mostly good, but if we make the change to `datasets` first then all users will keep getting deprecation warnings until we update the method in `transformers` and release a new version. 

I think we can follow your plan, but make the change to `transformers` first and wait for a new release before changing `datasets` - that way there are no visible warnings or API changes for users using `prepare_tf_dataset`. It also gives us more time to update the docs and try to move people to `prepare_tf_dataset` so they aren't confused by this!

Sounds good to me ! To summarize:
1. add the label renaming logic in `transformers` + release
1. show a warning in `to_tf_dataset` whevener a label is renamed automatically, saying that in the next major release this will be removed + minor release
1. after some time, do a major release 3.0.0 and remove label renaming completely in `to_tf_dataset`

Yep, that's the plan! 

@lhoestq Are you okay with me merging this for now? 

Can you remove `auto_rename_labels` ? I don't think it's a good idea to add it if the plan is to remove it later

Right now, the `auto_rename_labels` behaviour happens in all cases! Making it an option is the first step in the process of disabling it (and moving the functionality to `transformers`) and then finally deprecating it.