graph: interface: several fixes and code improves

[TaoLv]

1. JSON file serialization: removed the trailing whitespaces which may cause unintended edits when a JSON file is opened and closed with an editor (eg. vscode) configured with removing trailing whitespaces. Also added a newline at the end of the JSON file.
2. Moved the graph::serialize() method to cpp file so changing it or the JSON utility will not cause the whole graph component recompile (due to graph.hpp is almost included everywhere).
3. Backend interface: removed two lock guards which seem unnecessary.
4. fpmath_t: make the constructor explicit so we don't confuse it with fpmath_mode_t. At some places, we passed fpmath_mode_t to functions which requires fpmath_t as an argument.