Moving examples in docstrings of RobertaTokenizer and LongformerTokenizer to doc source files 
### Motivation 

When using the `# Copied from` mode of do-repeat-yourself, if there are interactive examples in the source file's docstring, e.g., [`[0, 31414, 232, 328, 2]` in RobertaTokenizer](https://github.com/huggingface/transformers/blob/c28d04e9e252a1a099944e325685f14d242ecdcd/src/transformers/models/roberta/tokenization_roberta.py#L118), then it is currently impossible for the destination file to have accurate interactive examples without having complicated replace patterns in the `copy-check` module. e.g., [`[0, 31414, 232, 2]` is the correct output of this example in `LongformerTokenizer`](https://github.com/huggingface/transformers/blob/main/src/transformers/models/longformer/tokenization_longformer.py#L127). 

For now, to pass the copy check we have copied over the output line from the Roberta model to Longformer.   

> The example should probably go in the doc source file instead, so we can copy without any worry. For now in this PR I'd leave it as the Roberta output as you did. Then we can do a follow-up PR where we change the doc source files for Roberta and Longformer and remove that example from the two docstrings (I can do it or you can, as you prefer!)

_Originally posted by @sgugger in https://github.com/huggingface/transformers/pull/19346#discussion_r988069210_
  
#self-assign<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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