# Replace autograd
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## Top

Questions to [David Rotermund](mailto:davrot@uni-bremen.de)

## Example 

```python
class FunctionalLinear(torch.autograd.Function):
    @staticmethod
    def forward(  # type: ignore
        ctx, input: torch.Tensor, weight: torch.Tensor, bias: torch.Tensor
    ) -> torch.Tensor:
        output = (weight.unsqueeze(0) * input.unsqueeze(1)).sum(dim=-1)

        if bias is not None:
            output = output + bias.unsqueeze(0)

        # ###########################################################
        # Save the necessary data for the backward pass
        # ###########################################################
        ctx.save_for_backward(
            input,
            weight,
            bias,
        )

        return output

    @staticmethod
    def backward(  # type: ignore
        ctx, grad_output: torch.Tensor
    ) -> tuple[torch.Tensor | None, torch.Tensor | None, torch.Tensor | None]:
        # ##############################################
        # Get the variables back
        # ##############################################
        (
            input,
            weight,
            bias,
        ) = ctx.saved_tensors

        # ##############################################
        # Default output
        # ##############################################
        grad_input: torch.Tensor | None = None
        grad_weight: torch.Tensor | None = None
        grad_bias: torch.Tensor | None = None

        grad_weight = grad_output.unsqueeze(-1) * input.unsqueeze(-2)

        if bias is not None:
            grad_bias = grad_output.detach().clone()

        grad_input = (grad_output.unsqueeze(-1) * weight.unsqueeze(0)).sum(dim=1)

        return grad_input, grad_weight, grad_bias
```

```python
class MyOwnLayer(torch.nn.Module):
    def __init__(
        self,
        in_features: int,
        out_features: int,
        bias: bool = True,
    ) -> None:
        super().__init__()

        assert in_features > 0
        assert out_features > 0

        self.in_features: int = in_features
        self.out_features: int = out_features

        self.weight = torch.nn.Parameter(
            torch.empty(
                (out_features, in_features),
            )
        )
        if bias:
            self.bias = torch.nn.Parameter(
                torch.empty(
                    out_features,
                )
            )
        else:
            self.register_parameter("bias", None)
        self.reset_parameters()

        self.functional_linear = FunctionalLinear.apply

    def reset_parameters(self) -> None:
        torch.nn.init.kaiming_uniform_(self.weight, a=math.sqrt(5))
        if self.bias is not None:
            fan_in, _ = torch.nn.init._calculate_fan_in_and_fan_out(self.weight)
            bound = 1 / math.sqrt(fan_in) if fan_in > 0 else 0
            torch.nn.init.uniform_(self.bias, -bound, bound)

    def forward(
        self,
        input: torch.Tensor,
    ) -> torch.Tensor:
        return self.functional_linear(input, self.weight, self.bias)

    def extra_repr(self) -> str:
        return f"in_features={self.in_features}, out_features={self.out_features}, bias={self.bias is not None}"
```