Complete implementation on CUDA of LayerNorm

[jordimas]

When you execute on CUDA of LayerNorm you get his message:

Generalized LayerNorm is currently not implemented on GPU

See: https://github.com/OpenNMT/CTranslate2/blob/master/src/ops/layer_norm_gpu.cu#L36

This task is to complete the implementation to allow to move the computation of these layers in Whisper to GPU:

https://github.com/OpenNMT/CTranslate2/blob/master/src/models/whisper.cc#L392

[BBC-Esq]

Hmmm...I'll send an initial pull request that you can tweak but have no idea how to do this...should be fun.