Removing noise model check in Simulator initialization

[asmuzsoy]

Addresses issue #4170 by removing NOISE_MODEL_LIKE check in sparse_simulator.py and updates tests accordingly.

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Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[asmuzsoy]

@googlebot I signed it!

[viathor]

Please merge the master branch back into this one.

[viathor]

I suppose for this PR we just want to verify that the simulator accepts the noise model, as opposed to raising an exception. However, we can make the test more useful by checking that the measurement falls into an interval (which ideally would be verified by a calculation independent of the code under test).

[viathor]

There is a class of bugs that make sum(result.measurements['0'])[0] equal to zero exactly (e.g. one such bug is the simulator accepting but ignoring the noise). These bugs would pass undetected by the test.

Suggestion: Replace identity with cirq.H(q) and change the assert to something like

assert 40 <= sum(result.measurements['0'])[0] < 60

Feel free to use tighter bounds, I just made those up. They should be gapped away from zero on the left and away from 100 on the right, though.

[asmuzsoy]

thanks for the suggestion, I just updated it!