!8168 fuse 2 quant_dtype_cast when srcAttr->srcT != dstAttr->dstT

Merge pull request !8168 from cjh9368/add_uint8_default_dtype

mindspore/lite/tools/converter/legacy_optimizer/fusion/quant_cast_fusion_pass.cc

@@ -62,7 +62,7 @@ STATUS QuantCastFusionPass::DoFusion(MetaGraphT *graph, const std::string &patte
   auto dstAttr = dstNode->primitive->value.AsQuantDTypeCast();
   MS_ASSERT(srcAttr != nullptr);
   MS_ASSERT(dstAttr != nullptr);
-  if (srcAttr->dstT != dstAttr->srcT || srcAttr->srcT != dstAttr->dstT) {
+  if (srcAttr->dstT != dstAttr->srcT) {
     MS_LOG(ERROR) << "srcNode and dstNode can not been fused";
     return RET_ERROR;
   }
@@ -73,11 +73,15 @@ STATUS QuantCastFusionPass::DoFusion(MetaGraphT *graph, const std::string &patte
     return status;
   }
 
+  if (srcAttr->srcT == dstAttr->dstT) {
     status = IsolateOneWayNode(graph, dstPath->nodeIdx);
     if (status != RET_OK) {
       MS_LOG(ERROR) << "IsolateOneWayNode failed, node: " << dstNode->name.c_str() << ", error: " << status;
       return status;
     }
+  } else {
+    dstAttr->srcT = srcAttr->dstT;
+  }
 
   return RET_OK;
 }