Fix custom operator imports in tests

[stavros11]

This fixes the issue of importing custom operators when the non-custom tensorflow operators are used on a machine that cannot compile custom operators. Particularly I identified two issues:

1. Custom operators were indeed imported by some gates that are only defined in cgates.py and not in gates.py (eg. the measurement gate). This was easy to fix because these gates do not actually use the custom operators.
2. The non-custom initial_state was not properly implemented when is_matrix=True.

[codecov]

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  Hits                    ?    11547           
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unittests	99.86% <0.00%> (?)

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[scarrazza]

Perfect, thanks!