Fix keepdims handling in layout_patterns.py

[fwsmit]

Description

Add default value with the same pattern as esp_ppq/executor/op/torch/default.py. I didn't check every operator, but I believe keepdims should always default to 1. Here's an example in the ONNX docs: https://onnx.ai/onnx/operators/onnx__ReduceMean.html.

Related

Testing

---

Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.