Implement ReduceL2 ONNX op

[sajattack]

Related: #322

Feature description

I tested one of my models with the new onnx frontend, and the ReduceL2 op appeared to be missing

Feature motivation

To run so-vits-svc voice conversion models in burn

https://drive.google.com/file/d/1pZ9bD75mDOVgPfdbsDPaoQKawneV0VrQ/view?usp=share_link

[antimora]

This project should be related: https://github.com/justinjohn0306/so-vits-svc-4.0-v2

[sajattack]

https://github.com/svc-develop-team/so-vits-svc