Fix pybind11 warnings in python_rpc_handler.cpp (pytorch#27284)

Summary:
Pull Request resolved: pytorch#27284

The warnings related to usage of the deprecated != operator. Instead
of checking the member field on every function call, we can check it
once, on construction of PythonRpcHandler.

Test Plan: Imported from OSS

Differential Revision: D17808213

Pulled By: pietern

fbshipit-source-id: 022c8f77f266942c49c55b1729e62dbb06262d77

torch/csrc/distributed/rpc/python_rpc_handler.cpp

@@ -4,13 +4,27 @@ namespace torch {
 namespace distributed {
 namespace rpc {
 
+namespace {
+
+py::object getFunction(const py::object& module, const char* name) {
+  py::object fn = module.attr(name);
+  TORCH_CHECK(
+      py::isinstance<py::function>(fn),
+      "attribute ",
+      name,
+      " is not a function");
+  return fn;
+}
+
+} // namespace
+
 PythonRpcHandler::PythonRpcHandler() {
   AutoGIL ag;
   py::object module =
       py::module::import("torch.distributed.internal_rpc_utils");
-  runUDFFunction_ = module.attr("run_python_udf_internal");
-  loadResultFunction_ = module.attr("load_python_udf_result_internal");
-  serializeFunction_ = module.attr("serialize");
+  runUDFFunction_ = getFunction(module, "run_python_udf_internal");
+  loadResultFunction_ = getFunction(module, "load_python_udf_result_internal");
+  serializeFunction_ = getFunction(module, "serialize");
 }
 
 PythonRpcHandler& PythonRpcHandler::getInstance() {
@@ -24,7 +38,6 @@ std::vector<char> PythonRpcHandler::generatePythonUDFResult(
     std::vector<torch::Tensor>& responseTensorTable) {
   AutoGIL ag;
   auto pargs = py::bytes(pickledPayload.data(), pickledPayload.size());
-  TORCH_CHECK(runUDFFunction_ != nullptr, "runUDFFunction_ is nullptr");
   py::tuple pres =
       serializeFunction_(runUDFFunction_(pargs, requestTensorTable));
   const auto& presStr = pres[0].cast<std::string>();
@@ -38,7 +51,6 @@ py::object PythonRpcHandler::loadPythonUDFResult(
     const std::vector<torch::Tensor>& tensorTable) {
   AutoGIL ag;
   auto pargs = py::bytes(pickledPayload.data(), pickledPayload.size());
-  TORCH_CHECK(loadResultFunction_ != nullptr, "loadResultFunction_ is nullptr");
   return loadResultFunction_(pargs, tensorTable);
 }