Use single rounding as default for TOSA lowering

[SaoirseARM]

• Use single rounding at rescale to int8
• Minor refactor of op_addmm to use shared functions

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/4591

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As of commit d65506588d7fd43c825ea80df17bbfa44a47b17b with merge base 9a4f32d56e33fef00d09c7a8e259ebeea66f1d78 (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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[SaoirseARM]

@pytorchbot label 'partner: arm'

[SaoirseARM]

@pytorchbot label ciflow/trunk

[pytorch-bot[bot]]

Can't add following labels to PR: ciflow/trunk. Please ping one of the reviewers for help.

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