Bump linux unittest runner to linux.2xlarge.memory

[GregoryComer]

Summary

Unittest jobs are dying since #15180. I'm not entirely sure why the jobs passed on the PR, but they are failing consistently since on trunk. Testing a runner size bump (2xlarge -> 2xlarge.memory) to see if that resolves it.

Example logs (from another PR). Note that the exit code (137 / SIGKILL) likely means OOM kill.

The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
...
Process completed with exit code 137.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15400

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❌ 2 New Failures, 2 Pending

As of commit 2a25e87 with merge base cd6f2e2 ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t ea9dfce90b929a454e4ac9b3a4845c8d8f01c4c0dca3d1a6fc5a84117116440f /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

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[GregoryComer]

Voxtral and samsung failures are due to running on a fork (I put up #15401 and #15402 to fix this).