Fix UNet implementation with arbitrary channel sizes (#243)

[vinayakjeet]

#243

Bug Description: The current UNet implementation in the Metalhead package has a limitation where it only works with input tensors of channel size 3. This restriction causes compatibility issues when users try to use UNet with input tensors of different channel sizes.

Patch Description: To address this limitation, I've modified the UNet implementation to support input tensors with arbitrary channel sizes. The UNet model can now handle input with varying dimensions

Test Case: using Metalhead UNet((128,128),1,3,Metalhead.backbone(DenseNet(121)))

This UNet model can process without any errors