The replacement happens in the codegen, which is launched during the build process, so it hasn’t happend yet at the line you printed extern_mod.
In addition, you should not see nuc_fpga_conv2d in Relay graph anyways, because nuc_fpga_conv2d is not a Relay op. The implementation of nuc_fpga_conv2d in your codegen is not registering an op to Relay. Instead, it just tells Relay when the op is offloaded to nuc_fpga, it should use the function you implemented (i.e., nuc_fpga_conv2d) to perform nn.conv2d.