Add test for proper TF input signatures
This is a relatively simple PR intended to check that all TF models have proper input signatures that match their inputs. I was calling the function `self._prune_signature` in a few places to verify this, which felt a bit hacky. This test should let us get rid of `self._prune_signature` by enforcing valid signatures for all models.

Edit: I'm also slipping in a typo fix (fine-tine -> fine-tune) I saw while I was running the tests

Double-edit: I'm also slipping in a fix to an incorrect indentation in the `test_dataset_conversion` test that was causing some unnecessary repetition
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