[bug] FileNotFoundError when loading robomimic config via module path

Describe the bug

When running imitation learning training with robomimic, the script fails to load the configuration file if the config path is given in the "module:file" format.

Error:

FileNotFoundError: [Errno 2] No such file or directory: 'isaaclab_tasks.manager_based.manipulation.stack.config.franka.agents:robomimic/bc_rnn_low_dim.json'

Root cause:
train.py directly uses open() on the module path string instead of resolving it into a filesystem path.
RSL-RL training already uses load_cfg_from_registry, which correctly parses "module:file" syntax.
As a result, imitation learning (robomimic) config loading fails.

Steps to reproduce

CUDA_VISIBLE_DEVICES=6 ./isaaclab.sh -p scripts/imitation_learning/robomimic/train.py   --task Isaac-Stack-Cube-Franka-IK-Rel-Skillgen-v0   --algo bc   --dataset ./datasets/generated_dataset_skillgen_cube_stack.hdf5

Log excerpt:

Traceback (most recent call last):
  File "scripts/imitation_learning/robomimic/train.py", line 449, in <module>
    main(args)
  File "scripts/imitation_learning/robomimic/train.py", line 372, in main
    with open(cfg_entry_point_file) as f:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: 'isaaclab_tasks.manager_based.manipulation.stack.config.franka.agents:robomimic/bc_rnn_low_dim.json'

System Info

• Commit: latest main (Nov 2025)
• Isaac Sim Version: 5.1
• OS: Ubuntu 22.04.5 LTS
• GPU: NVIDIA RTX 6000 Ada Generation
• CUDA: 12.8
• GPU Driver: 545.23.08

Additional context

• The same "module:file" config format works correctly in RSL-RL training.
• The issue appears only in scripts/imitation_learning/robomimic/train.py.
• Temporary workaround: manually resolve the module path to filesystem before opening.

Checklist

• I have checked that there is no similar issue in the repo
• I have checked that the issue is not in running Isaac Sim itself and is related to the repo

Acceptance Criteria

• Robomimic training should support "module:file" style config paths
• Config loading logic is unified with load_cfg_from_registry used in RSL-RL