Remove BasicAer shot limit (#7801)

* Fixing issue #7634

At this point, there is an arbitrary number for the maximum shots in the BasicAer simulator "65536". The solution proposed @kevinsung is to work with zero as the default value of max_shots. This solves the problem with the evaluation of max_shots in the assemble function "_parse_common_args" in qiskit/compiler/assembler.py.

* Update backend.py

* Update backend.py

* Update backend.py

* adding test

correcting some errors about where to add the test.

* Create remove-basicaer-shot-limit-b7cd4e6f6739885c.yaml

* Update releasenotes/notes/remove-basicaer-shot-limit-b7cd4e6f6739885c.yaml

Co-authored-by: Kevin J. Sung <kevinjefferysung@gmail.com>

* Remove test of shot limit

The test previously just ran `BasicAer` with two million shots on the
transpiled circuit to see if it encountered a limit.  This is simple
enough functionality (and unlikely enough to be broken accidentally)
that it can do without a test - the previous one took about 8 seconds,
which is a long time for testing a toggle.

Co-authored-by: Kevin J. Sung <kevinjefferysung@gmail.com>
Co-authored-by: Jake Lishman <jake.lishman@ibm.com>

qiskit/providers/basicaer/qasm_simulator.py

@@ -68,7 +68,7 @@ class QasmSimulatorPy(BackendV1):
         "conditional": True,
         "open_pulse": False,
         "memory": True,
-        "max_shots": 65536,
+        "max_shots": 0,
         "coupling_map": None,
         "description": "A python simulator for qasm experiments",
         "basis_gates": ["u1", "u2", "u3", "rz", "sx", "x", "cx", "id", "unitary"],

qiskit/providers/basicaer/statevector_simulator.py

@@ -48,7 +48,7 @@ class StatevectorSimulatorPy(QasmSimulatorPy):
         "conditional": True,
         "open_pulse": False,
         "memory": True,
-        "max_shots": 65536,
+        "max_shots": 0,
         "coupling_map": None,
         "description": "A Python statevector simulator for qobj files",
         "basis_gates": ["u1", "u2", "u3", "rz", "sx", "x", "cx", "id", "unitary"],

qiskit/providers/basicaer/unitary_simulator.py

@@ -69,7 +69,7 @@ class UnitarySimulatorPy(BackendV1):
         "conditional": False,
         "open_pulse": False,
         "memory": False,
-        "max_shots": 65536,
+        "max_shots": 0,
         "coupling_map": None,
         "description": "A python simulator for unitary matrix corresponding to a circuit",
         "basis_gates": ["u1", "u2", "u3", "rz", "sx", "x", "cx", "id", "unitary"],

releasenotes/notes/remove-basicaer-shot-limit-b7cd4e6f6739885c.yaml

@@ -0,0 +1,4 @@
+---
+upgrade:
+  - |
+    The measurement shot limit for the BasicAer backend has been removed.