text: 
  
  TITLE
  Add truncation_side option to tokenizers
  
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  # What does this PR do?
  
  As requested by #12909, it would be handy if one could also decide on whether to truncate sequences from the left instead of from the right. 
  
  As we already have a `padding_side` (which can be either left/right), it makes sense to also add a `truncation_side` (which by default is set to `"right"`, but users can initialize a tokenizer with `truncation_side` set to `"left"`).
  
  The test could possibly be improved (for which I'd like to get some help).
  
  Also requesting review from @patrickvonplaten since I've also added the option in `feature_extraction_sequence_utils.py`. 
  
  Regarding the fast tokenizers, I see `padding_side` is used [here](https://github.com/huggingface/transformers/blob/12e02e339f6d19218b36a30883188ea0254bc7e7/src/transformers/tokenization_utils_fast.py#L362). Should I define something similar for `truncation_side`?
  
  Fixes #12909 