Unify the implementation with multi-environments (wrap a single environment in a multi-environment with one envs) to greatly simplify the code.
This changed the behavior of single-environment.
Prior to this pr, for single environment, collector.collect(n_step=n) will step n steps.
After this pr, for single environment, collector.collect(n_step=n) will step m episodes until the steps are greater than n.
That is to say, collectors now always collect full episodes.
bfeffe1f97