Fix random seed initialization between runs in SabreSwap (#9127)

* Fix random seed initialization between runs in SabreSwap

This commit fixes an issue in the case no random seed is provided by the
user when initializing an instance of SabreSwap. The pass was previously
potentially reusing the random seed between runs even if no seed was
specified. This was an artifact of storing the initial seed as an
instance variable. Instead this commit just relies on the the Rust RNG
to initialize from entropy if no seed is specified.

* Remove unused numpy import

(cherry picked from commit dfbc738)

qiskit/transpiler/passes/routing/sabre_swap.py

@@ -15,7 +15,6 @@
 import logging
 from copy import copy, deepcopy
 
-import numpy as np
 import retworkx
 
 from qiskit.circuit.library.standard_gates import SwapGate
@@ -151,12 +150,7 @@ def __init__(self, coupling_map, heuristic="basic", seed=None, fake_run=False, t
             self._neighbor_table = NeighborTable(retworkx.adjacency_matrix(self.coupling_map.graph))
 
         self.heuristic = heuristic
-
-        if seed is None:
-            ii32 = np.iinfo(np.int32)
-            self.seed = np.random.default_rng(None).integers(0, ii32.max, dtype=int)
-        else:
-            self.seed = seed
+        self.seed = seed
         if trials is None:
             self.trials = CPU_COUNT
         else:

releasenotes/notes/fix-sabre-swap-random-seed-dcf3dace63042791.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed an issue with the :class:`~.SabreSwap` pass which would cause the
+    output of multiple runs of the pass without the ``seed`` argument specified
+    to reuse the same random number generator seed between runs instead of
+    using different seeds. This previously caused identical results to be
+    returned between runs even when no ``seed`` was specified.

src/sabre_swap/mod.rs

@@ -158,14 +158,17 @@ pub fn build_swap_map(
     neighbor_table: &NeighborTable,
     distance_matrix: PyReadonlyArray2<f64>,
     heuristic: &Heuristic,
-    seed: u64,
+    seed: Option<u64>,
     layout: &mut NLayout,
     num_trials: usize,
 ) -> (SwapMap, PyObject) {
     let run_in_parallel = getenv_use_multiple_threads();
     let dist = distance_matrix.as_array();
     let coupling_graph: DiGraph<(), ()> = cmap_from_neighor_table(neighbor_table);
-    let outer_rng = Pcg64Mcg::seed_from_u64(seed);
+    let outer_rng = match seed {
+        Some(seed) => Pcg64Mcg::seed_from_u64(seed),
+        None => Pcg64Mcg::from_entropy(),
+    };
     let seed_vec: Vec<u64> = outer_rng
         .sample_iter(&rand::distributions::Standard)
         .take(num_trials)