Support py.typed

[peteboothroyd]

It would be nice if ml-metadata exposed its type information to clients, like:

tensorflow/tfx#3981

See PEP-561 for more details.

Concretely this just means adding an empty file called py.typed at the root of the package.

[BrianSong]

Hi peteboothroyd,

Thanks for raising up this issue.

To clarify, are you having some problems without ml_metadata exposing its type information?

[peteboothroyd]

The problem is that although ml-metadata has type annotated its public api this is not checked by clients which use tools such as mypy, so clients aren't getting the benefits provided by type checking their usage of ml-metadata.

[BrianSong]

Acknowledged, let's keep this issue open and add type checking support in the next one or two release.