Temporarily drop down number of trials for level1 and level2 (#4099)

In #3999 we increased the number of trials, but the global benefit from
doing this isn't clear. With the pending release weighing the increased
run time for more trials against this, this commit temporarily reverts
the number of trials for level 1 and level 2 back to their previous
setting of 20. When we get to the bottom of #4094 and have a fix or
change for that we can increase the number of trials again.

Related #4094

qiskit/transpiler/preset_passmanagers/level1.py

@@ -119,7 +119,7 @@ def _swap_condition(property_set):
     if routing_method == 'basic':
         _swap += [BasicSwap(coupling_map)]
     elif routing_method == 'stochastic':
-        _swap += [StochasticSwap(coupling_map, trials=50, seed=seed_transpiler)]
+        _swap += [StochasticSwap(coupling_map, trials=20, seed=seed_transpiler)]
     elif routing_method == 'lookahead':
         _swap += [LookaheadSwap(coupling_map, search_depth=4, search_width=4)]
     else:

qiskit/transpiler/preset_passmanagers/level2.py

@@ -114,7 +114,7 @@ def _swap_condition(property_set):
     if routing_method == 'basic':
         _swap += [BasicSwap(coupling_map)]
     elif routing_method == 'stochastic':
-        _swap += [StochasticSwap(coupling_map, trials=100, seed=seed_transpiler)]
+        _swap += [StochasticSwap(coupling_map, trials=20, seed=seed_transpiler)]
     elif routing_method == 'lookahead':
         _swap += [LookaheadSwap(coupling_map, search_depth=5, search_width=5)]
     else: