Refine layerwise non-mutating calibration

[realAsma]

Stacked on #1571.

Summary:

• Add calib_mutates_weights gating for non-mutating layerwise calibration.
• Skip layer weight checkpoint/writeback for quantizer-state-only calibration.
• Keep FSDP2/Accelerate writeback conditional and improve layerwise progress reporting.

Testing:

• Pre-commit hooks passed during commit.

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[realAsma]

@sugunav14 here is an easy perf improvement for layerwise FSDP2