Add GPT-2 text generation example

[robertknight]

Add a text generation / completion example using GPT-2. This also includes the start of some utilities to implement the generation loop for auto-regressive transformer models.

The initial version uses simple greedy sampling from the output logits. As a result, completions are prone to being repetitive. I plan to replace this with top-K sampling soon.

Example usage:

$ cargo run -r -p rten-examples --bin gpt2 gpt2_medium_onnx/model.rten gpt2_medium_onnx/tokenizer.json "Once fine day, in the middle of"

Output:

Once fine day, in the middle of the night, I was sitting in my room, watching TV, when I heard a knock on the door. I opened it and there was a man