Add environment parse function that supports default value (pytorch#8…

…5563)

We use "-2" to represent an unset environment variable.
Now adding a util function to attach default value if environment variable is unset.
Pull Request resolved: pytorch#85563
Approved by: https://github.com/rohan-varma, https://github.com/H-Huang

torch/csrc/distributed/c10d/ProcessGroupNCCL.cpp

@@ -623,8 +623,8 @@ ProcessGroupNCCL::ProcessGroupNCCL(
       at::cuda::getNumGPUs() != 0,
       "ProcessGroupNCCL is only supported with GPUs, no GPUs found!");
   blockingWait_ = parseEnvVarFlag(NCCL_BLOCKING_WAIT);
-  asyncErrorHandling_ =
-      static_cast<ErrorHandlingMode>(parseEnvVarInt(NCCL_ASYNC_ERROR_HANDLING));
+  asyncErrorHandling_ = static_cast<ErrorHandlingMode>(
+      parseEnvVarIntDefault(NCCL_ASYNC_ERROR_HANDLING, 0));
   desyncDebug_ = parseEnvVarFlag(NCCL_DESYNC_DEBUG) ||
       (dist_debug_level_ >= DebugLevel::Detail);
 

torch/csrc/distributed/c10d/Utils.hpp

@@ -88,6 +88,13 @@ inline int parseEnvVarInt(const char* envVarName) {
   return C10D_ENV_NOT_SET;
 }
 
+inline int parseEnvVarIntDefault(const char* envVarName, int defaultVal) {
+    int val = parseEnvVarInt(envVarName);
+    if (val == C10D_ENV_NOT_SET)
+      return defaultVal;
+    return val;
+}
+
 inline bool parseEnvVarFlag(const char* envVarName) {
     int val = parseEnvVarInt(envVarName);
     if (val == 1) {