Add NCCL_PROTO=^LL128 to recommended NCCL environment variables (#253)

Author: msimberg

docs/software/communication/nccl_env_vars

@@ -10,6 +10,9 @@ export NCCL_NET="AWS Libfabric"
 # https://docs.nvidia.com/deeplearning/nccl/user-guide/docs/env.html#nccl-net-gdr-level-formerly-nccl-ib-gdr-level
 export NCCL_NET_GDR_LEVEL=PHB
 export NCCL_CROSS_NIC=1
+# Starting with nccl 2.27 a new protocol (LL128) was enabled by default, which
+# typically performs worse on Slingshot. The following disables that protocol.
+export NCCL_PROTO=^LL128
 # These `FI` (libfabric) environment variables have been found to give the best
 # performance on the Alps network across a wide range of applications. Specific
 # applications may perform better with other values.