Improve observation manager for observation groups with history_length != None.

[pculbertson-bdai]

Description

This PR makes some low-level changes to the observation_manager, particularly w/r/t how it handles ObservationGroups with non-zero history_length. It first changes how the observation group shapes are computed - which fixes a shape bug in the env.observation_space for this group downstream.

It also changes the handling of flattened obs groups to construct tensors in a way that the history dimension is preserved.

Fixes #1957.

Type of change

Unsure if this is a bugfix or breaking change, since it would change the ordering of elements in observation groups with history_length != None. Policies trained with the old ordering would not work - but the proposed ordering seems more in line with the shapes described in the documentation.

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

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