Fix resume checkpoint for tf2 codepath

[almostimplemented]

Cheap, localized solution to the bug described in #1911

For the TF2 codepath, provide additional two methods to the DummyModule: load_checkpoint and get_trainable_variables.

[almostimplemented]

Hm, maybe these can be added directly to the DummyModule construction, down at def framework (line 83 in original).