Addressing review comments from Umang

- rename then/else_shapes -> then/else_lens
- change assert for shape size to reusing throw()
- return lengths in handle_empty_branch
- use handle_empty_branch return value for lens update
- use std::prev() instead of predecriment operator for modifying nodes

src/onnx/parse_if.cpp

@@ -68,7 +68,15 @@ struct parse_if : op_parser<parse_if>
         auto then_out_shapes = then_mdl->get_output_shapes();
         auto else_out_shapes = else_mdl->get_output_shapes();
 
-        assert(then_out_shapes.size() == else_out_shapes.size());
+        auto throw_shapes = [&]() {
+            MIGRAPHX_THROW("PARSE_IF: " + info.name +
+                            " then and else sub_graphs must compatible shapes ");
+        };
+
+        if(then_out_shapes.size() != else_out_shapes.size())
+        {
+            throw_shapes();
+        }
 
         // Must have the same type for both if/else blocks by onnx spec
         // Add exception for empty constant scalars
@@ -82,15 +90,11 @@ struct parse_if : op_parser<parse_if>
 
         if(not then_out_shapes.at(0).dynamic() && not else_out_shapes.at(0).dynamic())
         {
-            auto then_shape   = then_out_shapes.at(0).lens();
-            auto else_shape   = else_out_shapes.at(0).lens();
-            auto throw_shapes = [&]() {
-                MIGRAPHX_THROW("PARSE_IF: " + info.name +
-                               " then and else sub_graphs must compatible shapes ");
-            };
+            auto then_lens   = then_out_shapes.at(0).lens();
+            auto else_lens   = else_out_shapes.at(0).lens();
 
             // Throw error if both branches have zero output shapes. Not possible for static inputs
-            if(then_shape.empty() && else_shape.empty())
+            if(then_lens.empty() && else_lens.empty())
             {
                 throw_shapes();
             }
@@ -98,30 +102,29 @@ struct parse_if : op_parser<parse_if>
             auto handle_empty_branch = [](module_ref& mdl, const shape& out_shape) {
                 auto outline_ins = mdl->add_outline(out_shape);
                 mdl->replace_return({outline_ins});
+                return out_shape.lens();
             };
 
             // Handle one empty branch by setting output identical to the other
             // need to update the then_shape before we do further checks
-            if(then_shape.empty())
+            if(then_lens.empty())
             {
-                handle_empty_branch(then_mdl, else_out_shapes.at(0));
-                then_shape = else_shape;
+                then_lens = handle_empty_branch(then_mdl, else_out_shapes.at(0));
             }
-            else if(else_shape.empty())
+            else if(else_lens.empty())
             {
-                handle_empty_branch(else_mdl, then_out_shapes.at(0));
-                else_shape = then_shape;
+                else_lens = handle_empty_branch(else_mdl, then_out_shapes.at(0));
             }
             else
             {
                 // check equivilant length dims, and (x1,x2,.., xn, 1) == (x1,x2,..,xn)
-                int dim_delta = abs((static_cast<int>(then_shape.size() - else_shape.size())));
+                int dim_delta = abs((static_cast<int>(then_lens.size() - else_lens.size())));
 
                 if(dim_delta <= 1)
                 {
                     // make sure dims are equivalent in static shapes
-                    if(not equal(then_shape.begin(), then_shape.end(), else_shape.begin()) &&
-                       not equal(else_shape.begin(), else_shape.end(), then_shape.begin()))
+                    if(not equal(then_lens.begin(), then_lens.end(), else_lens.begin()) &&
+                       not equal(else_lens.begin(), else_lens.end(), then_lens.begin()))
                     {
                         throw_shapes();
                     }
@@ -130,22 +133,22 @@ struct parse_if : op_parser<parse_if>
                                                 const std::vector<size_t>& out_shape) {
                         auto convert_ins = mdl->add_instruction(
                             make_op("unsqueeze", {{"axes", {out_shape.size() - 1}}}),
-                            --(--mdl->end()));
+                            std::prev(std::prev(mdl->end())));
                         mdl->replace_return({convert_ins});
-                        mdl->remove_instruction({--convert_ins});
+                        mdl->remove_instruction({std::prev(convert_ins)});
                     };
 
-                    auto last_then = *(--(then_shape.end()));
-                    auto last_else = *(--(else_shape.end()));
+                    auto last_then = *(std::prev(then_lens.end()));
+                    auto last_else = *(std::prev(else_lens.end()));
 
                     // Find which dim to unsqueeze
-                    if((then_shape.size() < else_shape.size()) && (last_else == 1))
+                    if((then_lens.size() < else_lens.size()) && (last_else == 1))
                     {
-                        unsqueeze_last_op(then_mdl, else_shape);
+                        unsqueeze_last_op(then_mdl, else_lens);
                     }
-                    else if((then_shape.size() > else_shape.size()) && (last_then == 1))
+                    else if((then_lens.size() > else_lens.size()) && (last_then == 1))
                     {
-                        unsqueeze_last_op(else_mdl, then_shape);
+                        unsqueeze_last_op(else_mdl, then_lens);
                     }
                 }
                 else