Create cirq_google.InternalGate

[NoureldinYosri]

This is the first step towards porting internal gates to Cirq.

[dstrain115]

LGTM with some nits.

[NoureldinYosri]

@maffoo let me know if this makes sense

[maffoo]

Discussed over chat with @NoureldinYosri. I want to be sure we think carefully about how to handle units here, and I actually lean toward putting units in names (e.g. InternalGate("CouplerDelayZ", ..., delay_ns=1, ...)) because we've seen internally that dealing with plain numbers results in significant performance gains compared to allocating lots of objects with units (InternalGate("CouplerDelayZ", ..., delay=Value(1, "ns"), ...)). Also, we don't generally benefit much from the flexibility of doing unit conversions since most parameters for gates have one "natural" unit and the unit name is mostly for documentation. All that said, it doesn't need to block moving ahead on this PR, so LGTM.

[NoureldinYosri]

@dstrain115 @maffoo concerning the problem of passing units it's a symptom of a larger problem which is that we won't be able to pass values that in themselves are instances of internal classes. adding a suffix to the argument name opens a can of worms for who to say that the gate doesn't have an argument with the modified name and it will be a nightmare to support and maintain. while exposing some internal classes defeats the purpose of this project.

The most suitable solution is to support internal classes the same way we support internal gates, that's why I introduced cirq_gate.InternalClass

please take another look

[NoureldinYosri]

I discarded the last change after talking with @dstrain115 since we won't support units in the first version of this work.