I have VS2017 (15.5.4) Professional already installed and today I installed .Net Core SDK 2.1.4 but I cannot see any Core templates in the New Project dialog in VS.
For .NET Core 2.1, you need Visual Studio version 15.7 or higher. (check it from Help menu > About Microsoft Visual Studio). So, first ensure you have update your visual studio.
Now, to add .NET Core 2.1 (or other newer templates) to visual studio, you need to install the Latest .Net Core SDK from here.