Fix the gold index issue in fetching logprob

[fangleai]

In story generation task, the original code fetches incorrect index of "logprob", leading to incorrect perplexity evaluation.

Originally: tgt_in = tgt[:-1] log_probs, attn = ... ... gold = tgt_in

Edited: tgt_in = tgt[:-1] log_probs, attn = ... ... gold = tgt[1:]