Conditionally support expand_copy in XNNPACK delegate

[GregoryComer]

Summary

Expand is sometimes used to to express an unsqueeze operation (inserting singleton dims) - one motivating example is in MobileBERT (link). This PR adds a pass to XNNPACK to conditionally transform expand_copy into view_copy, as long as the expand is used solely to add singleton dimensions. It also updates the partitioner with the appropriate constraint.

Note that this PR is stacked on top of #7959, as it relies on these changes (delegation of view_copy).

Test plan

Added test coverage in backends/xnnpack/test/ops/test_expand.py and backends/xnnpack/test/passes/test_expand_to_view_pass.py. These cover end-to-end delegation and pass-specific logic.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/7960

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❌ 1 New Failure

As of commit 7ce711f with merge base e78ed83 ():

NEW FAILURE - The following job has failed:

• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t ad98bd9df08790eb9d98e3c26ae69bd80ac9822d08a5bc66f7b2a59f21eb7514 /exec failed with exit code 1

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