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In particular, the following assumptions are FALSE in general:
tokenize(context + continuation) = tokenize(context) + tokenize(continuation)
len(tokenize(context + continuation)) = len(tokenize(context)) + len(tokenize(continuation))
tokenize(context + continuation)[:len(tokenize(context))] = tokenize(context)

So we need to tip-toe around the problem by being careful with how we do it.

In particular, using Fast is not just for performance; while behavour of GPT2Tokenizer differs across Transformers 2 and 3, GPT2TokenizerFast doesn't.

It's ok if subclasses don't implement download, no-op default is ok