Add support for exporting an architecture to TFLite

Adding support for an unsupported architecture

If you wish to export a model whose architecture is not already supported by the library, the PR #813 Adds support for ResNet can be used as a reference.

You can make sure tests pass for the new my_new_modeltype model type by running:

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pytest tests/exporters/tflite/test_*.py -k "my_new_modeltype" -s --exitfirst