Make GRU-based RNNs exportable in RSL RL

[WT-MM]

Description

Adds correct forward pass for GRU-based recurrent policies in the isaaclab_rl/rsl_rl/exporter.py script.

See issue for more details

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[WT-MM]

Would appreciate any thoughts or details I may have missed here