Fixes skrl train/play script configurations when using the `--agent` argument and rename agent configuration variable  (#3643)

# Description

This PR address the following points:
* Fix skrl train/play script configuration when using the `--agent`
argument

    Example:

    ```bash
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

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applicable. -->

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