Clarify Lambada Task

[StellaAthena]

When OpenAI created GPT-2, they also created a custom, non-standard lambada evaluation dataset. OpenAI also changed the metric for evaluation by counting number of times the last BPE token is predicted incorrectly instead of the last word. This produces a huge difference in performance score, totally over 10%. They used this easier version of Lambada for evaluating GPT-2 as well. For more details, see here and here.

According to @jon-tow, we implement the openai version, not the standard version. We should implement both, and call them lambada_standard and lambada_openai respectively. In particular, we should not implement a task called lambada because years after the fact this is still causing widespread confusion and we want to force the user to pay attention to it.