Make use of MONAI blocks in diffusion_model_unet.py

[Warvito]

I think GEGLU and FeedForward components from diffusion_model_unet.py

GenerativeModels/generative/networks/nets/diffusion_model_unet.py

Line 53 in 7dcdc9a

class GEGLU(nn.Module):

GenerativeModels/generative/networks/nets/diffusion_model_unet.py

Line 71 in 7dcdc9a

class FeedForward(nn.Module):

are already implemented on MONAI core as MLPBlock

https://github.com/Project-MONAI/MONAI/blob/17529e7ae0a53db68f9ace7a130da2b611650cc3/monai/networks/blocks/mlp.py#L22

It might be necessary further investigation.

[Warvito]

The current version of MLPBlock is not compatible with the Feed Forward block used inside the Diffusion model. Waiting for Project-MONAI/MONAI#5853 to see if it will be possible to have an unified solution.