Cortex_m backend: Add MV3 and lstm test

[AdrianLundell]

Adding as proof of concept, skipping until functionally supported. Note that LSTM is one op in CMSIS-NN, which is why it is added in the op folder.

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15393

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❌ 2 New Failures, 2 Unrelated Failures

As of commit bc0e964 with merge base 4efd79c ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 08102721d0732bbb346dd17ff9620a5bba6bbde79c14e451cf6a9aa0bd4b692b /exec failed with exit code 1
• Test Metal Backend / export-voxtral-metal-artifact / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)

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