export() cleanup
This pull request **introduces 16 alerts** when merging c90bef348f061e638f8d9e14c6d04ff9b1e4a0c6 into 24674db0995736ece1d55515d300f5ef725bf4ba - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-12332d4be2b2234f7a6bf7ff7304577ac875b65b)

**new alerts:**

* 16 for Unused import This pull request **introduces 10 alerts** when merging c3cfe2aab1f0636ad7b7563fd8b9255a41f89ed8 into 7f633b7953a2e6ce798e74515bc99276d6fb4a3b - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-7e327ee6fed8f374100d4598fc0f24ba417d05e8)

**new alerts:**

* 10 for Unused import Well, those changes were not only for cleanup - I only did refactoring because I did not want to make same changes in many files. There were functional issues affecting the clients - input_example and output_example parameters to export() would be rejected. Essentially, no user would be able to export a model with custom fixed size inputs - like they can do with modules. I think it's essential feature and we should not be losing it. Another lack of functionality was that most export's did not support TS format at all (only one I think was). Re QA effort - there is no need to re-run re-export of those models that were exported - my changes would not change the result of those. 
As for regression - they are covered by unit tests. re-route to main Closing, re-routed to main.
Signed-off-by: Boris Fomitchev <bfomitchev@nvidia.com>