Lesson6-rnn notebook RuntimeError when calling fit function

I tried to run Lesson6-rnn notebook and encountered:

RuntimeError: Input batch size 64 doesn’t match hidden batch size 1

after calling get_next() function in the RNN Loop section:

fit(m, md, 4, opt, F.nll_loss)

I was wondering if you guys would help me figure out how to solve this. Thank you!