add warning to let the user know that the `__call__` method is faster than `encode` + `pad` for a fast tokenizer
Author: SaulLu
Pull request: yes

# What does this PR do?

In this PR I propose to specify in the docstring of the `pad` method and to log a warning when the pad method is called with a fast tokenizer. The goal is to encourage the user to use the `__call__` method to encode and pad their input at the same time since it will be faster then encoding and then padding it with the method `pad` (because it is not using the rust backend).


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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_18693). All of your documentation changes will be reflected on that endpoint.

Thanks a lot for the advice @LysandreJik :hugs: ! Indeed, since this attribute `deprecation_warnings` exists, I have no reason to prefer `warnings.warn` to `logger.warning`. I made the change in https://github.com/huggingface/transformers/pull/18693/commits/775002fbd2c0c67001182d7ce07e5efc4a1b9c80.