Staging `auto_reset` as option

[sotetsuk]

Currently, auto_reset feature is provided as experimental but as it's actually a MUST to use auto_reset in vectorized env, we'll implement it to public API.

• auto_reset="replace" : To overwrite the final state with the initial state when an episode ends.
• auto_reset="manual" : To allow the agent to continue acting on the terminal state and transition to the initial state.
• auto_reset=None : To represent no automatic reset.