Enable type-checking for transpiler.py and test_transpiler.py (#308)

* Enable type-checking for `transpiler.py` and `test_transpiler.py`

This commit adds type annotations to the transpiler module and its
corresponding tests, enabling full type-checking with `mypy` while
preserving existing functionality.

Author: thierry-martinez

graphix/sim/base_backend.py

@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 from abc import ABC, abstractmethod
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Literal
 
 import numpy as np
 
@@ -22,6 +22,7 @@
     from graphix import command
     from graphix.fundamentals import Plane
     from graphix.measurements import Measurement
+    from graphix.parameter import ExpressionOrFloat
     from graphix.simulator import MeasureMethod
 
 
@@ -160,8 +161,8 @@ def _op_mat_from_result(vec: tuple[float, float, float], result: bool, symbolic:
 
 
 def perform_measure(
-    qubit: int, plane: Plane, angle: float, state, rng, pr_calc: bool = True, symbolic: bool = False
-) -> npt.NDArray:
+    qubit: int, plane: Plane, angle: ExpressionOrFloat, state, rng, pr_calc: bool = True, symbolic: bool = False
+) -> Literal[0, 1]:
     """Perform measurement of a qubit."""
     vec = plane.polar(angle)
     if pr_calc:

graphix/sim/statevec.py

@@ -281,7 +281,7 @@ def tensor(self, other: Statevec) -> None:
         total_num = len(self.dims()) + len(other.dims())
         self.psi = np.kron(psi_self, psi_other).reshape((2,) * total_num)
 
-    def cnot(self, qubits):
+    def cnot(self, qubits) -> None:
         """Apply CNOT.
 
         Parameters