updating nccl interface documantation

Author: caiomcbr

apps/nccl/README.md

@@ -44,3 +44,22 @@ The table below lists all NCCL APIs (v2.21). We may cover more APIs in the futur
 | ncclRecv                 | X         |
 | ncclRedOpCreatePreMulSum | X         |
 | ncclRedOpDestroy         | X         |
+
+### Executor Support
+
+The executor is a versatile tool designed to specify how mscclpp executes algorithms. Currently, only the allReduce operation allows for algorithm customization. The following environment variables can be managed:
+
+- ALLREDUCEPKT_IP_JSON_FILE: Specifies the path to the JSON file that defines the algorithm for small-sized, in-place operations.
+- ALLREDUCEPKT_OP_JSON_FILE: Specifies the path to the JSON file that defines the algorithm for small-sized, out-of-place operations.
+- ALLREDUCE_IP_JSON_FILE: Specifies the path to the JSON file that defines the algorithm for larger-sized, in-place operations.
+- ALLREDUCE_OP_JSON_FILE: Specifies the path to the JSON file that defines the algorithm for larger-sized, out-of-place operations.
+- ALLREDUCE_SMALL_MSG_BOUNDARY: Defines the size threshold at which the algorithm will switch between fallback code and the customized algorithm for small messages.
+- ALLREDUCE_LARGE_MSG_BOUNDARY: Defines the size threshold at which the algorithm will switch between the customized algorithm for small messages and that for larger messages.
+
+| <center>Decision Flowchart for Message Size-Based Algorithm Execution |
+|-------------------------------|
+| <img src="../.././docs/figs/size_boundary_diagram.png" alt="MSCCL++ Abstractions" style="width: 800px;"/> |
+
+This is an example of executing the interface with the executor:
+``` bash
+mpirun -np 8 -x ALLREDUCEPKT_IP_JSON_FILE=/root/azure-mscclpp/nccl/test/execution-files/allreducepacket.json -x ALLREDUCE_IP_JSON_FILE=/root/azure-mscclpp/nccl/test/execution-files/allreducesm.json -x ALLREDUCE_SMALL_MSG_BOUNDARY=16K -x ALLREDUCE_LARGE_MSG_BOUNDARY=1M ./apps/nccl/test/nccl_api_test