set custom_nll_loss_op attr ignoreIndex to str (#42596)

set attr ignoreIndex type to string for custom_nllloss_op

部分 cheery-pick of #42534

paddle/fluid/platform/device/ipu/popart_canonicalization/other_ops.cc

@@ -57,14 +57,21 @@ Node *checkpointoutput_handler(Graph *graph, Node *node) {
 Node *custom_nll_loss_handler(Graph *graph, Node *node) {
   auto *op = node->Op();
   auto reduction = BOOST_GET_CONST(int, op->GetAttr("reduction"));
-  auto ignoreIndex = BOOST_GET_CONST(int, op->GetAttr("ignoreIndex"));
+  auto ignoreIndex = BOOST_GET_CONST(std::string, op->GetAttr("ignoreIndex"));
   auto inputIsLogProbability =
       BOOST_GET_CONST(bool, op->GetAttr("inputIsLogProbability"));
+  if (ignoreIndex == "None") {
     return CreateBaseOp(graph, node, "popart_nllloss_v2", node->inputs,
                         node->outputs,
                         {{"reduction", reduction},
-                       {"ignoreIndex", ignoreIndex},
                          {"inputIsLogProbability", inputIsLogProbability}});
+  } else {
+    return CreateBaseOp(graph, node, "popart_nllloss_v2", node->inputs,
+                        node->outputs,
+                        {{"reduction", reduction},
+                         {"ignoreIndex", std::atoi(ignoreIndex.c_str())},
+                         {"inputIsLogProbability", inputIsLogProbability}});
+  }
 }
 
 Node *identity_handler(Graph *graph, Node *node) {