Inserts a placeholder for a tensor that will be always fed. Placeholder is used to build the model.
dw::Placeholder in(dw::Shape{32, 100});
dw::Placeholder out = dw::Linear(50, "linear_0")(in);
out = dw::ReLU("relu_1")(out);
out = dw::Linear(10, "linear_2")(out);
out = dw::Softmax("probs")(out);