Remove unused exported tensor constants after Cortex-M lowering

[rascani]

Cortex-M layout conversion leaves original constant placeholders that FX dead-code elimination preserves. Add a separate RemoveUnusedConstantsPass to retire unused input specs and storage in linear time. Include it after constant lifting in both default pass lists, after padding fusion and scratch sizing.

Lift constants exactly once through the pass list before pruning, avoiding generated buffer-name collisions. Remove the unconditional final lifting call: explicit custom lists define the complete pipeline and can select no cleanup, lifting only, or lifting followed by pruning. The small lifting adapter makes the existing utility usable by the standard ExportedProgram pass runner.

Preserve user inputs, mutation and gradient targets, preserved module-call signatures, and state shared with other entry points. The existing EXIR pass and other backend defaults remain unchanged. Eleven focused Python tests passed, including buffer-name collision and all three custom-list behaviors.

Authored with AI assistance from Codex.

[rascani]

Stack from ghstack (oldest at bottom):

• -> Remove unused exported tensor constants after Cortex-M lowering #22862
• Cortex-M: fuse SAME padding into quantized convolutions #22861
• Cortex-M: size scratch buffers in a dedicated pass #22860
• Cortex-M: avoid redundant convolution and pooling work #22859

[pytorch-bot]

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