If you want to modify your model definition, you can just add another layer before self.conv1(inputs). If you don’t want to modify it, you can do something like
class MyModule(ActorCritic):
    def __init__(self, **kwargs):
        super(MyModule, self).__init__(**kwargs)
        self.new_conv = nn.Conv2d(3, 3, 1)

    def forward(self, inputs):
        inputs, (hx, cx) = inputs
        x = F.elu(self.new_conv(inputs))
        return super(MyModule, self).forward((x, (hx, cx)))

Or variations of that