Add tests for tokenization truncation. Does it truncate to the left or to the right?

[thejaminator]

In our function batch_tokenize_gpt2, it truncates the text if its too long.
I forgot if the default huggingface behavior truncates the left tokens or the right tokens.

IMO we should always truncate the right tokens.
This is so that not every text that hits the context limit has an eos token at the end of it.

i.e. suppose you have a context length of 3 words. And you are tokenizing The quick brown fox
It should get tokenized to

The quick brown

rather than

quick brown EOS

And if you tokenized Hello there
It doesn't get truncated and becomes

Hello there EOS

We should write some tests to enforce this behavior for batch_tokenize_gpt2