Hi @bhadresh-savani, as far as I can tell the problem seems to lie with your find_sublist_indices function, not on the availability of a fast tokenizer.
One simple thing to try: can you pass a slice of examples to your convert_to_features function, e.g.
convert_to_features(train_dataset[:3])

I’m not sure whether this will solve the problem, but perhaps your find_sublist_indices is expected a list of lists which is what you’ll get from the slice.
I also noticed that your convert_to_features function is quite different to the prepare_train_features in the tutorial - what happens if you try the latter with your tokenizer?
If that doesn’t work, then you might be able to use the old run_qa.py script that doesn’t rely on fast tokenizers: transformers/examples/legacy/question-answering at master · huggingface/transformers · GitHub 3
Lewis