Fixes gate label import

[purva-thakre]

Fixes import of gate labels when gates were moved to operations folder. See related comment.

Tested with examples in this notebook.

The notebook also needs a separate PR to change how gates are imported + deprecation warning in In [62]. This will be added later.

Following code block is for future reference in relation to In [62] :

<ipython-input-60-0c4e722f5dae>:3: DeprecationWarning: `np.complex` is a deprecated alias for the builtin `complex`. To silence this warning, use `complex` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.complex128` here.
Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
  mat = np.zeros((4, 4), dtype=np.complex)

[hodgestar]

We shouldn't add these _ constants and functions to __all__ I don't think and we don't need to import _GATE_NAME_TO_LABEL in circuit.py at all.

[purva-thakre]

we don't need to import _GATE_NAME_TO_LABEL in circuit.py at all

We do actually. It's used in circuit.py. I tried running one of the notebooks with _gate_label and that caused some errors.

[hodgestar]

$ ag _GATE_NAME_TO_LABEL .
src/qutip_qip/operations/gates.py
171:            if (name in _GATE_NAME_TO_LABEL) and (arg_value is not None):
226:_GATE_NAME_TO_LABEL = {
263:    if name in _GATE_NAME_TO_LABEL:
264:        gate_label = _GATE_NAME_TO_LABEL[name]

I'm not seeing it used anywhere other than in gates.py? Could you provide a link to where else it is used?

[purva-thakre]

Sorry, I was confused between _gate_label and _GATE_NAME_TO_LABEL. You are right. Only _gate_label has to be imported.

Although, for this issue Boxi wanted a separate PR. Feel free to close this PR if that's changed.

[BoxiLi]

Oh, I was asking for a separate PR mainly because the decomposition is a new feature and this is a bug fix. Mixing them might cause some confusion if someone checks the history one day. As #66 is all about bug fixing and cleaning, I can just merge that one and saves you the trouble of updating it for _GATE_NAME_TO_LABEL. But thank you anyway!

[hodgestar]

@purva-thakre Thank you for noticing this. I came across it independently and addressed it in a slightly different in #66. If #66 is merged, I'll close this PR. Apologies for not finding this PR before I fixed the bug in mine.

[hodgestar]

#66 also adds a test for .latex_code().

[purva-thakre]

@purva-thakre Thank you for noticing this. I came across it independently and addressed it in a slightly different in #66. If #66 is merged, I'll close this PR. Apologies for not finding this PR before I fixed the bug in mine.

No problem @hodgestar