Fix Adam subgroup inconsistency

[st-bang97]

Fix CPUAdam same-step subgroup drift in ZeRO-3 (#7819)

This PR ports the fix from #7820 to the latest DeepSpeed version.

It makes Adam_Optimizer::IncrementStep idempotent for repeated calls at the same logical step and avoids unnecessary recomputation when the step has not changed.

ZeRO-3/SuperOffload can invoke multiple subgroup updates within a single logical step on a shared native optimizer object. The previous logic mixed multiply and recompute paths, producing non-bit-identical bias-correction metadata across subgroup calls.

This change aligns the step-transition logic in both the CPU and XPU headers, clarifies first-step and non-sequential-step behavior, and prevents unnecessary work on repeated same-step updates.

It also adds CPUAdam regression tests covering subgroup-style repeated same-step updates through both step_subgroup() and step() with parameter swapping.

[delock]

Hi @st-bang97 can you fix formatting? Thanks!

[st-bang97]

@delock maybe i fix the formatting right? if not, could you tell me which code i need to fix the format?

[delock]

@delock maybe i fix the formatting right? if not, could you tell me which code i need to fix the format?

Hi @st-bang97 you can refer to this link for the formatting error.
Also use the following method to check formatting error, ususally yapf and flake8 will fix formatting automatically during this process:

• Verify changed files pass pre-commit checks before committing: pre-commit run --files <changed_files>.

[st-bang97]

@delock Thanks for your help. I edited the format of PR.