Add missing tokenizer tests - Longformer

[tgadeliya]

What does this PR do?

This PR add tests for Longformer tokenizer copying tests from Roberta tokenizer's test suite, because those tokenizers are absolutely identical.

Fixes #16627

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@SaulLu @LysandreJik

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[tgadeliya]

I read discussion in merged tokenizers' tests PRs and post Don't Repeat Yourself* on HF blog and I manually add "the copying mechanism". But I don't understand how it is work, so I tried not to change copied test code from Roberta tokenizer tests. If code modification is not a problem, I would like to add some minor changes, e.g. delete commented code and split big test into smaller one.
Could describe "copying mechanism" works in more details?

[SaulLu]

Thanks a lot for working on this @tgadeliya!!

As far as I know, there are no identified "practices" for this case (cc @LysandreJik in case you have another opinion). Nevertheless, if changes are relevant, they are obviously welcome. For example, it is possible to indicate the changes made as here:

transformers/src/transformers/models/deberta/modeling_deberta.py

Lines 308 to 309 in d95a32c

	# Copied from transformers.models.bert.modeling_bert.BertIntermediate with Bert->Deberta
	class DebertaIntermediate(nn.Module):

If the differences are too long to list perhaps the message can just explain why it diverged from the originally copied and pasted code.

Does this help you?

[tgadeliya]

@SaulLu, Sorry for the late reply. Summer is ending :)

Thanks for your comment. Now it is clear for me. Actually, I came to the conclusion, that code cleaning not so necessary considering all pros and cons. So this PR can be reviewed and merged

[SaulLu]

Sounds great to me! Can you just merge/rebase on main so we can merge your PR?

[tgadeliya]

@SaulLu I refreshed this PR, so now it is ready to merge

[SaulLu]

Thanks @tgadeliya 🤗