use nullptr to address clang-tidy

Author: Protonu Basu

torch/csrc/jit/tensorexpr/cuda_codegen.cpp

@@ -934,7 +934,7 @@ void CudaCodeGen::call(const std::vector<CallArg>& args) {
 void CudaCodeGen::CompileToNVRTC(
     const std::string& code,
     const std::string& func_name) {
-  CUcontext pctx = 0;
+  CUcontext pctx = nullptr;
   AT_CUDA_DRIVER_CHECK(nvrtc().cuCtxGetCurrent(&pctx));
   // Note: hacked at::DeviceGuard since at::DeviceGuard was failing to work
   // properly in some scenarios