add PreTrainedTokenizers link

The base classes PreTrainedTokenizer and PreTrainedTokenizerFast implements the common methods for encoding string inputs in model inputs and instantiating/saving python tokenizers either from a local file or directory or from a pretrained tokenizer provided by the library (downloaded from HuggingFace’s AWS S3 repository).

PreTrainedTokenizer and PreTrainedTokenizerFast thus implements the main methods for using all the tokenizers:

PreTrainedTokenizer `(transformers.PreTrainedTokenizer) <https://huggingface.co/transformers/main_classes/tokenizer.html#transformers.PreTrainedTokenizer>`_ thus implements the main methods for using all the tokenizers:

- Tokenizing (spliting strings in sub-word token strings), converting tokens strings to ids and back, and encoding/decoding (i.e. tokenizing + convert to integers),

For more details on the base tokenizers which the DeepChem tokenizers inherit from, please refer to the following: `HuggingFace tokenizers docs <https://huggingface.co/transformers/main_classes/tokenizer.html>`_

SmilesTokenizer

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