Fix num_head_channels in 3d_ddpm_tutorial

[Warvito]

Since just the third level of the network has attention mechanisms, it will be better to define the num_head_channels as different from 0 for just this level instead of having values for the other levels as well.

GenerativeModels/tutorials/generative/3d_ddpm/3d_ddpm_tutorial.py

Line 152 in 7d02598

num_head_channels=[256, 256, 512],

Changing to num_head_channels=[0, 0, 512], will not affect the network but it will be less confusing for the users.