intercept SIGUSR2 in RCCL

add support for intercepting SIGUSR2 in RCCL. This signal will
not terminate the execution of the application, but print the stacktrace
of the process that the signal was sent to instead.

src/misc/signals.cc

@@ -58,6 +58,10 @@ void sig_handler(int signum)
   free (strings);
 #endif
 
+  if (signum == SIGUSR2) {
+    return;
+  }
+
   exit (-1);
 }
 
@@ -69,7 +73,7 @@ void RegisterSignalHandlers()
   {
     INFO(NCCL_INIT, "Enabling custom signal handler");
 
-    std::vector<int> signalsToCatch = {SIGILL, SIGBUS, SIGFPE, SIGSEGV};
+    std::vector<int> signalsToCatch = {SIGILL, SIGBUS, SIGFPE, SIGSEGV, SIGUSR2};
 
     for (auto signum : signalsToCatch)
     {