Update cc12m_256x256.yaml with diffusion_config addition and use null as a correct version of None

[tolgacangoz]

Proposes to fix #10

1. This pull request updates the cc12m_256x256.yaml and cc12m_1024x1024.yaml files by adding a diffusion_config section because samplers are created with diffusion_config.sampler_config:
   

   ml-mdm/ml_mdm/diffusion.py

   Line 94 in 3695917

   self.sampler = samplers.Sampler(diffusion_config.sampler_config)

   
   

   ml-mdm/ml_mdm/diffusion.py

   Line 287 in 3695917

   self.sampler = samplers.NestedSampler(diffusion_config.sampler_config)

2. Also, diffusion_config.no_use_residual is needed:
   

   ml-mdm/ml_mdm/diffusion.py

   Line 262 in 3695917

   if not self.diffusion_config.no_use_residual:

3. None in the file is read by 'None' -as a string.

@jlukecarlson