fix fill_constant bug and add int64->int32 cast test=develop (#2566)

- fix fill_constant bug.
- cast op support int64_t->int32_t

lite/kernels/arm/cast_compute.cc

@@ -56,6 +56,12 @@ void CastCompute::Run() {
     float* out_data = param.Out->mutable_data<float>();
     std::transform(
         x_data_begin, x_data_end, out_data, TransOp<unsigned char, float>);
+  } else if (param.in_dtype == 3 && param.out_dtype == 2) {
+    const int64_t* x_data_begin = param.X->data<int64_t>();
+    const int64_t* x_data_end = x_data_begin + param.X->numel();
+    int32_t* out_data = param.Out->mutable_data<int32_t>();
+    std::transform(
+        x_data_begin, x_data_end, out_data, TransOp<int64_t, int32_t>);
   } else {
     LOG(FATAL) << "other has not been implemented";
   }

lite/operators/fill_constant_op.cc

@@ -51,9 +51,8 @@ class FillConstantOp : public OpLite {
     param_.shape_tensor_list = {};
 
     std::vector<std::string> input_arg_names = opdesc.InputArgumentNames();
-    if (std::find(input_arg_names.begin(),
-                  input_arg_names.end(),
-                  "ShapeTensor") != input_arg_names.end()) {
+    if (opdesc.HasInput("ShapeTensor") &&
+        !opdesc.Input("ShapeTensor").empty()) {
       auto args = opdesc.Input("ShapeTensor");
       auto* var = scope->FindVar(args.front());
       param_.shape_tensor = var->GetMutable<lite::Tensor>();