[Relax] Expose BlockBuilder's Analyzer instance in Python

[oskar-inceptron]

It is currently not possible to access a BlockBuilder's Analyzer instance from Python code, which would be useful e.g. when converting from front-end representations to Relax.

This PR addresses that by adding a get_analyzer method to the Python BlockBuilder class. Behind the scenes, it uses the same FFI and code paths as when creating a standalone Python Analyzer, the only difference being where the underlying C++ instance originates from. To avoid object lifetime issues in Python code (which would be surprising given that it is a memory managed language), a C++ BlockBuilder's Analyzer field is now a shared pointer.

[Hzfengsy]

cc @tqchen

[tqchen]

thanks @oskar-inceptron If we want to go towards this directly, a better approach is to make Analyzer an Object, so we can use ObjectPtr for this