implement nn.Module flytekit type transformer

[cosmicBboy]

don't require torch as a package dependency, just try importing it in unionml/__init__.py and registering the type transformers if torch is installed.

Try out specifying flytekit.plugins entrypoint in the setuptools config to make sure that these unionml plugins can work with flytekit plugin system

[cosmicBboy]

this will be handled flyteorg/flytekit#1032