Implement AdaptiveAvgPool2d for Candle

[ozoromo]

Feature description

Currently trying to use AdaptiveAvgPool2d with the Candle backend resuilts in a crash with the error: adaptive_avg_pool2 is not supported by Candle

I would like to see it implemented if possible.

Feature motivation

It seems there was already an effort to add implement this for all backends in #541, however candle was left out, possibly due to it not being added at that point.

Also with Candle being among the fastest backends, it makes sense to have it available,

[louisfd]

We have no control over existing Candle operations, but you could submit an issue over their repo: https://github.com/huggingface/candle

[antimora]

A new issue should be filed with Candle and come back to update Candle backend.