docs: Fill in missing Doctrings for models

[soumik12345]

Action Items

• [x] Add references to the doctrings for SimpleGate.
• [x] Elaborate docstrings for ChannelAttention with references and more details.
• [x] Elaborate docstrings for SimplifiedChannelAttention with references and more details.
• [x] Elaborate docstrings for NAFBlock with references and more details.
• [x] Add references and parameters to the docstrings for PixelShuffle.
• [x] Add references and parameters to the docstrings for UpScale.
• [x] Add references to the docstrings for NAFNet.

[Samarendra109]

UpScale is just a convenience class that I had added. It uses pixelshuffle only. The authors of the original paper has used pytorch sequential module instead. I don't think we need to add reference for that block.