[Relax][Frontend][ONNX] Fix LpPool conversion

[napronald]

Summary

Fixes two issues in the Relax ONNX LpPool converter:

• Computes |x|^p instead of x^p, matching the official ONNX reference implementation.
• Passes the TVM dtype directly to relax.const, avoiding a NumPy dtype conversion failure.

Minimal reproduce

python -m pytest \
  tests/python/relax/test_frontend_onnx.py::test_pool \
  -vv

conversion failed with:

ValueError: Could not convert T.float32 to a NumPy dtype

[gemini-code-assist]

Caution

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[tlopex]

LGTM