Arm backend: Add PTE diff stage to Arm test pipeline (pytorch#23056)

Adds an Arm pipeline stage helper for comparing the generated
`to_executorch` PTE against a reference PTE. When the PTEs differ,
append TOSA delegate diagnostics showing payload sizes, op counts, ops
unique to each side, and the first operator type mismatch.
- Allow atol,rtol for tensor drift

Signed-off-by: Saoirse Stewart <saoirse.stewart@arm.com>

Author: SaoirseARM