Minor fixes and benchmarking of auto-stake extrinsic.

[shamil-gadelshin]

Description

This PR contains minor fixes and benchmarking for set_coldkey_auto_stake_hotkey extrinsic.

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run cargo fmt and cargo clippy to ensure my code is formatted and linted correctly
• 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
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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Additional Notes

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