Created by: TomorrowIsAnOtherDay

This PR:

1. deletes previous files relative to tensorboard to avoid potential chaos in tensorboard files.
2. creates tensorboard files after the first call of tensorboard functions. We used to create a writer when importing parl.utils.tensorboard.

When users run the following code:

from parl.utils import logger
from parl.utils import tensorboard
import numpy as np

parl will not create any tensorbard files.