Fix skrl train/play script configurations when using the --agent argument and rename agent configuration variable

[Toni-SM]

Description

This PR address the following points:

• Fix skrl train/play script configuration when using the --agent argument

  Example:

  python scripts/reinforcement_learning/skrl/train.py --task Isaac-Cart-Double-Pendulum-Direct-v0 --headless --agent skrl_mappo_cfg_entry_point

  Error:

  [INFO]: Parsing configuration from: isaaclab_tasks.direct.cart_double_pendulum.cart_double_pendulum_env:CartDoublePendulumEnvCfg
  [INFO]: Parsing configuration from: /home/toni/Documents/RL/toni_IsaacLab/source/isaaclab_tasks/isaaclab_tasks/direct/cart_double_pendulum/agents/skrl_mappo_cfg.yaml
  [INFO] Logging experiment in directory: /home/toni/Documents/RL/toni_IsaacLab/logs/skrl/cart_double_pendulum_direct
  Error executing job with overrides: []
  Traceback (most recent call last):
    File "/home/toni/Documents/RL/toni_IsaacLab/source/isaaclab_tasks/isaaclab_tasks/utils/hydra.py", line 101, in hydra_main
      func(env_cfg, agent_cfg, *args, **kwargs)
    File "/home/toni/Documents/RL/toni_IsaacLab/scripts/reinforcement_learning/skrl/train.py", line 156, in main
      log_dir = datetime.now().strftime("%Y-%m-%d_%H-%M-%S") + f"_{algorithm}_{args_cli.ml_framework}"
                                                                  ^^^^^^^^^
  NameError: name 'algorithm' is not defined
  

• Replace STATES by OBSERVATIONS when defining skrl's agent configuration model inputs to ensure a smooth and error-free transition when the new mayor version of skrl gets released. In such mayor version OBSERVATIONS and STATES have different value/usage.

Type of change

• Bug fix (non-breaking change which fixes an issue)