Add dialogue data collator tests

[CloseChoice]

Add dialogue data collator unit test. Things to note on this PR:

• is it correct that we mask the last occurance of <|endoftext|> of the assistant? See the example in the test, there will be one occurance where we have this token and one where there is none. See the todo in the code.
• I built a dummy tokenizer from the pythia-70m one using tokenizer = old_tokenizer.train_new_from_iterator(training_iter, vocab_size) to keep the size minimal. Just trained on the text that appears in the test.

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