Enable linearly changing min/max steps in guidance

[bennyguo]

This PR is related to #175 . I made the following changes:

• add set_min_max_steps to all guidance (as in #175)
• in the update_step hook of all guidance, use C to get the parsed value of min_step and max_step if they're specified by a tuple of four numbers
• add min_step and max_step to the return dict of the guidance, so that we could monitor the actual used value in tensorboard / wandb