conv-transpose2d backwards support

I was making a UNet and ran into the lack of backwards support for conv_transpose2d.

https://github.com/huggingface/candle/blob/563a79afa1c6a5ad75ab88aef805f79fb1559f73/candle-core/src/backprop.rs#L312-L314

Also looking into alternatives like Upsampling2d like support but didn't find that option. Was wondering what the timeline would be like to get this supported? Either conv_transpose2d backwards or upsampling2d for both.

Thank you.

Start at candle-core/src/backprop.rs around lines 312-314 and inspect how conv_transpose2d currently lacks backward support. Compare the requested alternatives in the issue, then define done as adding backward support for conv_transpose2d or providing the proposed upsampling2d support, with appropriate validation in the existing project tests.

Written by the indexing model from the issue text.