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 allows XNNPACK to conditionally lower expand_copy, as long as the expand is used solely to add singleton dimensions.

Test plan

Added test coverage in backends/xnnpack/test/ops/test_expand.py.

[pytorch-bot]

🔗 Helpful Links

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 144b550 with merge base d886373 ():

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

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

• pull / android / run-emulator (gh) (trunk failure)
  Timeout waiting for emulator to boot.

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[mcr229]

looks good let's just address those comments

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