[GSoC] Better lazy_imports usability

[Conchylicultor]

Part1: Currently, lazy imports feels counter intuitive and confusing. It would be nice to improve the API to allow something like:

with tfds.core.lazy_imports():
  import some_module

It would patch the default module import:

• If the module is installed (e.g. tested with try/except ImportError): Import the module as standard
• If the import isn't installed: Replace the module by a dummy ModuleType which raise error when access attribute (use __getattr__):

with tfds.core.lazy_imports():
  import some_module

some_module.some_function()  # Raise the lazy import error (please install some_module)

[vijayphoenix]

Working on this!