Factor out import of common tflite.Operator in tflite frontend. (#5355)

* Restructure imports in tflite frontend.

These python modules are needed for every tflite file parsed.
Factorize out imports of the common most ones.

Now that the import of operator is common, asserts can be commonized.

Loses 473 lines of duplication.

* Only restrict to tflite.Operator