Handle step key correctly in checkpoint save with --optimizer-cpu-offload

[ahmadki]

What does this PR do ?

Skip 'step' key before shape assertion as it's a 0-dim tensor handled separately via param_groups, not as part of the gradient buffer.

Fixes #2775

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[Phlip79]

/ok to test 6927efc

[asolergi-nv]

#2775 Mentions that the issue arises when using --optimizer-cpu-offload and --use-precision-aware-optimizer. Do we know why this works correctly without using these flags?

[asolergi-nv]

/ok to test 3430450