Add support for NCCL2. The configure script asks for what version of NCCL to...

Add support for NCCL2. The configure script asks for what version of NCCL to use. The default is still NCCL 1 from GitHub. If the user chooses NCCL 2, it asks for the install directory.

The nccl_configure.bzl generates two different BUILD files based on the chose NCCL version. For NCCL 1, it aliases to the existing 'nccl_archive' http_repo on GitHub. For NCCL 2, it creates a target containing the NCCL 2 library and headers from the chosen install directory.

PiperOrigin-RevId: 191718007