Reduce default TX size for NCCL environment variables (#294)

32k is too high for libfabric and the default (1k) is used instead. Use
the real max: 16k.

Author: msimberg

docs/software/communication/nccl_env_vars

@@ -17,7 +17,7 @@ export NCCL_PROTO=^LL128
 # performance on the Alps network across a wide range of applications. Specific
 # applications may perform better with other values.
 export FI_CXI_DEFAULT_CQ_SIZE=131072
-export FI_CXI_DEFAULT_TX_SIZE=32768
+export FI_CXI_DEFAULT_TX_SIZE=16384
 export FI_CXI_DISABLE_HOST_REGISTER=1
 export FI_CXI_RX_MATCH_MODE=software
 export FI_MR_CACHE_MONITOR=userfaultfd