@arvindmohan
I just find answer.
Actually, my question was  Should I initialize hidden state in every epoch?
If I put init_hidden() method in forward() It will initialize hidden state in every epoch.
What I found is, If I have independent sequence data like text, I have to initialize hidden state in every epoch.
But If I have dependent sequence data like stock price or something. Hidden state should not be initialized because hidden state would be keep used in the data.