tests/ep: disable NCCL HeartbeatMonitor to silence mpirun shutdown noise

Set TORCH_NCCL_ENABLE_MONITORING=0 before importing torch.distributed.
The barrier+destroy_process_group finally block (afbdcd6a) suffices
under torchrun, but under mpirun rank 0 (the TCPStore server) can exit
before non-zero ranks finish teardown, and the background heartbeat
thread polls the store and logs 'recvValue failed / Connection was
likely closed'. Disabling the monitor outright is safe for short-lived
bench runs.

test/python/ext/ep/test_low_latency_multirank.py

@@ -35,6 +35,12 @@ import os
 import random
 import sys
 
+# Disable ProcessGroupNCCL's HeartbeatMonitor before importing torch.distributed.
+# It runs in a background thread polling the TCPStore; under mpirun, rank 0
+# (the store server) can exit before non-zero ranks finish teardown, producing
+# noisy 'recvValue failed / Connection was likely closed' stack traces.
+os.environ.setdefault("TORCH_NCCL_ENABLE_MONITORING", "0")
+
 import torch
 import torch.distributed as dist