Separates per-step termination and last-episode termination bookkeeping

[ooctipus]

Description

This PR fixes the issue where get_done_term returned last episode value rather than current step value.

This PR realizes values used for get_term should be different from that used for logging, and mixed useage leads to non-intuitive behavior.

using per-step value for logging leads to overcounting and undercounting reported in #2977
using last-episode value for get_term leads to misalignment with expectation reported in #3720

Fixes #2977 #3720

Type of change

The logging behavior remains mostly the same as #3107, and and also got rid of the weird overwriting behavior(yay).
I get exactly the same termination curve as #3107 when run on Isaac-Velocity-Rough-Anymal-C-v0

Here is a benchmark summary with 1000 steps running Isaac-Velocity-Rough-Anymal-C-v0 with 4096 envs

Before #3107:

| termination.compute | 0.229 ms|
| termination.reset | 0.007 ms|

PR #3107:

| termination.compute | 0.274 ms|
| termination.reset | 0.004 ms|

This PR:

| termination.compute | 0.258 ms|
| termination.reset | 0.004 ms|

We actually see improvement, this is due to the fact that expensive maintenance of last_episode_value is only computed once per compute(#3107 computes last_episode_value for every term)

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

Screenshots

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Checklist

• I have read and understood the contribution guidelines
• 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