Replace double occurrences as the last step
This PR fixes an issue with the SPM converters (ALBERT and XLNet) where it would replace some characters by whitespace - after removing double whitespace occurrences. This meant that if double whitespace were to appear thanks to this replacement, they would be kept until the end of encoding, leading to a mismatch between SentencePiece and `tokenizers`.

Fixes https://github.com/huggingface/transformers/issues/11358
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