Fixed some minor bugs in play.py

[Homerain]

• The original play.py attempted to implement a complex phase trajectory visualization but was missing the required variables and functions (e.g., for the M20), leading to runtime errors. This version replaces it with a simplified pure trajectory drawing approach, initializes only the minimal necessary variables, and works correctly (e.g., for the M20; when playing in multi‑environment settings, trajectory tracking is displayed only on one environment's robot to avoid visual clutter).

• At this stage, the priority is to ensure that training and playback work properly. Therefore, this modification focuses on that goal. Other related features can be added later by overwriting this file once the original contributor has completed their implementation.

• Before the next official update, please verify that the code runs without errors and check that comments are consistent in language.