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 any templates: Make sure VS has the necessary components, make sure that the cross platform development workload is installed through the Visual Studio Installer.
The SDK 2.1.* only contains the 2.0.* runtimes. At the time of writing, there has been no public preview release for .NET Core 2.1.
You can download 2.2.0 SDK previews from https://github.com/dotnet/cli.
Note that the naming may change in the near future in order to avoid confusion and keep the SDK and Runtime major and minor versions in sync - see https://github.com/dotnet/designs/pull/29.