Draft: [Feat] Support dequantization of an exported model

Context

Related Issue

Changes

Features

• Add Cast operator implementation (Export CPP);
• ignore node attribute is no longer replaced by the exclude_unwanted_producers() function, allowing more flexibility;
• set_nodes_datatypes() function now keeps the float tensors to float type instead of turning them into int32;

Test

• Add Cast unit test;

Doc

• Add a dequant LeNet example;