Add Supported for PhasedXZGate

[MichaelBroughton]

There's a new and obscure gate in Cirq! The PhasedXZGate. We should add support for this gate inside of TensorFlow Quantum. Like all gates this means:

1. Adding rules to serializer.py
2. Implementing the gate in qsim
3. Adding a rule in tfq_ps_decompose_op.cc
4. Hooking the gate into all existing python fuzz tests and supported_gates function in util.py.

Do @jaeyoo or @zaqqwerty have any interest in taking this on ?

[jaeyoo]

I am interested in 1 & 3.

[MichaelBroughton]

Sounds good. I can help with 2 and 4 :)

[jaeyoo]

Thanks. Let me open a PR for 1 in this week

[MichaelBroughton]

After talking with the people working on qsim it looks like this may be looked after for us already. This should come for free when we switch over to using the OSS version of qsim

[jaeyoo]

Thanks a lot, then I will not work on it :)

[MichaelBroughton]

This gate is in now.