ColossalAI/docs/model.md

# Define your own parallel model

Let's say that you have a huge MLP model with billions of parameters and its extremely large hidden layer size makes it
impossible to fit into a single GPU directly. Don't worry, ColossalAI is here to help you sort things out. With the help of ColossalAI, 
you can write your model in the familiar way in which you used to write models for a single GPU, while ColossalAI automatically 
splits your model weights and fit them perfectly into a set of GPUs. We give a simple example showing how to write a simple 
2D parallel model in the Colossal-AI context.

## Write a simple 2D parallel model

```python
from colossalai.nn import Linear2D
import torch.nn as nn

class MLP_2D(nn.Module):

    def __init__(self):
        super().__init__()
        self.linear_1 = Linear2D(in_features=1024, out_features=16384)
        self.linear_2 = Linear2D(in_features=16384, out_features=1024)

    def forward(self, x):
        x = self.linear_1(x)
        x = self.linear_2(x)
        return x
```

## Use pre-defined model

For the sake of your convenience, we kindly provide you in our Model Zoo with some prevalent models such as *BERT*, *VIT*, 
and *MLP-Mixer*. Feel free to customize them into different sizes to fit into your special needs.