feat: SB3 optimizations (3x faster)

[araffin]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context.
List any dependencies that are required for this change.

Implement part of #1769 (optimization)

This is a breaking change because the fast variant is now enabled by default.

I also improve sb3 training script, fixed loading of normalization and fixed the humanoid hyperparameters to be similar to rsl-rl, so we can compare apples to apples in terms of training speed.

I will probably open another PR for the rest of the proposals.

Type of change

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

With respect to testing, how do you run a single test?
and is there anything I should add?

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

[araffin]

@Mayankm96 failure on the CI seems to be due to a timeout, probably unrelated to this PR?