Arm backend: Support dyn. shapes in rewrite_slice

[oscarandersson8218]

Preserve symbolic and proxy slice bounds when normalizing start and end indices.

Build dynamic slice sizes from shape operators and relax TOSA SLICE validation for non-static start and size values.

cc @digantdesai @freddan80 @per @zingo @mansnils @Sebastian-Larsson @robell @rascani

[pytorch-bot]

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❌ 1 New Failure, 2 Pending

As of commit 5992b17 with merge base c2e8d64 ():

NEW FAILURE - The following job has failed:

• Cadence Build & Test / Resolve CI docker image / resolve (gh)
  ##[error]Refusing to check out fork pull request code from a 'pull_request_target' workflow. This workflow runs with the base repository's GITHUB_TOKEN, secrets, default-branch cache scope, and runner access. Fetching and executing a fork's code in that trusted context commonly leads to "pwn request" vulnerabilities. To opt in, review the risks at https://gh.io/securely-using-pull_request_target and set 'allow-unsafe-pr-checkout: true' on the actions/checkout step.

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[oscarandersson8218]

Unrelated failures.