Repository: PennyLaneAI/pennylane (python)

Issue:
[BUG] Hamiltonian expansion with dynamic shots fails
### Expected behavior

I expect that a Hamiltonian expectation value is expanded if the execution occurs with finite shots. 

### Actual behavior

If the device has `shots=None` upon construction, but the circuit is dynamically executed with finite shots, Hamiltonian expansion does not occur.

This happens because the batch transformation occurs before the device shots are set in `qml.exeucte`.  We need to change the order for which these things occur.

### Additional information

_No response_

### Source code

```shell
@qml.qnode(qml.device('default.qubit', wires=2), diff_method=None)
def circuit():
    return qml.expval(2.0 * qml.PauliX(0))

circuit(shots=20)
```


### Tracebacks

```shell
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
Cell In [4], line 1
----> 1 circuit(shots=20)

File ~/Prog/pennylane/pennylane/qnode.py:665, in QNode.__call__(self, *args, **kwargs)
    661     self._update_original_device()
    663     return res
--> 665 res = qml.execute(
    666     [self.tape],
    667     device=self.device,
    668     gradient_fn=self.gradient_fn,
    669     interface=self.interface,
    670     gradient_kwargs=self.gradient_kwargs,
    671     override_shots=override_shots,
    672     **self.execute_kwargs,
    673 )
    675 if old_interface == "auto":
    676     self.interface = "auto"

File ~/Prog/pennylane/pennylane/interfaces/execution.py:637, in execute(tapes, device, gradient_fn, interface, mode, gradient_kwargs, cache, cachesize, max_diff, override_shots, expand_fn, max_expansion, device_batch_transform)
    631         return batch_fn(
    632             qml.interfaces.cache_execute(
    633                 batch_execute, cache, return_tuple=False, expand_fn=expand_fn
    634             )(tapes)
    635         )
    636     with qml.tape.Unwrap(*tapes):
--> 637         res = qml.interfaces.cache_execute(
    638             batch_execute, cache, return_tuple=False, expand_fn=expand_fn
    639         )(tapes)
    641     return batch_fn(res)
    643 if gradient_fn == "backprop" or interface is None:

File ~/Prog/pennylane/pennylane/interfaces/execution.py:206, in cache_execute.<locals>.wrapper(tapes, **kwargs)
    202         return (res, []) if return_tuple else res
    204 else:
    205     # execute all unique tapes that do not exist in the cache
--> 206     res = fn(execution_tapes.values(), **kwargs)
    208 final_res = []
    210 for i, tape in enumerate(tapes):

File ~/Prog/pennylane/pennylane/interfaces/execution.py:131, in cache_execute.<locals>.fn(tapes, **kwargs)
    129 def fn(tapes: Sequence[QuantumTape], **kwargs):  # pylint: disable=function-redefined
    130     tapes = [expand_fn(tape) for tape in tapes]
--> 131     return original_fn(tapes, **kwargs)

File /Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/contextlib.py:79, in ContextDecorator.__call__.<locals>.inner(*args, **kwds)
     76 @wraps(func)
     77 def inner(*args, **kwds):
     78     with self._recreate_cm():
---> 79         return func(*args, **kwds)

File ~/Prog/pennylane/pennylane/_qubit_device.py:624, in QubitDevice.batch_execute(self, circuits)
    621     self.reset()
    623     # TODO: Insert control on value here
--> 624     res = self.execute(circuit)
    625     results.append(res)
    627 if self.tracker.active:

File ~/Prog/pennylane/pennylane/_qubit_device.py:390, in QubitDevice.execute(self, circuit, **kwargs)
    388     results = self._collect_shotvector_results(circuit, counts_exist)
    389 else:
--> 390     results = self.statistics(circuit.observables, circuit=circuit)
    392 if not circuit.is_sampled:
    394     if len(circuit.measurements) == 1:

File ~/Prog/pennylane/pennylane/_qubit_device.py:743, in QubitDevice.statistics(self, observables, shot_range, bin_size, circuit)
    739 for obs in observables:
    740     # Pass instances directly
    741     if obs.return_type is Expectation:
    742         # Appends a result of shape (num_bins,) if bin_size is not None, else a scalar
--> 743         results.append(self.expval(obs, shot_range=shot_range, bin_size=bin_size))
    745     elif obs.return_type is Variance:
    746         # Appends a result of shape (num_bins,) if bin_size is not None, else a scalar
    747         results.append(self.var(obs, shot_range=shot_range, bin_size=bin_size))

File ~/Prog/pennylane/pennylane/devices/default_qubit.py:534, in DefaultQubit.expval(self, observable, shot_range, bin_size)
    530 # intercept other Hamiltonians
    531 # TODO: Ideally, this logic should not live in the Device, but be moved
    532 # to a component that can be re-used by devices as needed.
    533 if observable.name in ("Hamiltonian", "SparseHamiltonian"):
--> 534     assert self.shots is None, f"{observable.name} must be used with shots=None"
    536     self.map_wires(observable.wires)
    537     backprop_mode = (
    538         not isinstance(self.state, np.ndarray)
    539         or any(not isinstance(d, (float, np.ndarray)) for d in observable.data)
    540     ) and observable.name == "Hamiltonian"

AssertionError: Hamiltonian must be used with shots=None
```


### System information

```shell
On master.
```


### Existing GitHub issues

- [X] I have searched existing GitHub issues to make sure the issue does not already exist.

Pull request:
batch transform occurs with the correct number of shots
Fixes #3368 .

Currently, device batch transformations occur with the number of shots originally specified on the device, not the number of shots used for the execution.  In the cases where the device has no shots but the execution does, this leads to errors if we are taking the expectation value of a Hamiltonian.

```python
@qml.qnode(qml.device('default.qubit', wires=2), diff_method=None)
def circuit():
    return qml.expval(2.0 * qml.PauliX(0))

circuit(shots=20)
```

This quick fix merely overrides the device shots for the duration of `batch_transform`.

This bug highlights the need to:
* Remove the shots specification and move it to some sort of "execution config" data class
* Make device preprocessing depend on the execution config data class

Patch:
diff --git a/doc/releases/changelog-dev.md b/doc/releases/changelog-dev.md
index f67b7ceed..240278bcd 100644
--- a/doc/releases/changelog-dev.md
+++ b/doc/releases/changelog-dev.md
@@ -44,8 +44,15 @@
   attributes were modified.
   [#3292](https://github.com/PennyLaneAI/pennylane/pull/3292)
 
+* If the device originally has no shots but finite shots are dynamically specified, Hamiltonian
+  expansion now occurs.
+  [(#3369)](https://github.com/PennyLaneAI/pennylane/pull/3369)
+
 <h3>Contributors</h3>
 
 This release contains contributions from (in alphabetical order):
 
-Juan Miguel Arrazola, Utkarsh Azad, Soran Jahangiri,
\ No newline at end of file
+Juan Miguel Arrazola
+Utkarsh Azad
+Soran Jahangiri
+Christina Lee
diff --git a/pennylane/interfaces/execution.py b/pennylane/interfaces/execution.py
index c90b557ce..77f8ece07 100644
--- a/pennylane/interfaces/execution.py
+++ b/pennylane/interfaces/execution.py
@@ -238,7 +238,7 @@ def _execute_new(
     cache=True,
     cachesize=10000,
     max_diff=1,
-    override_shots=False,
+    override_shots: int = False,
     expand_fn="device",
     max_expansion=10,
     device_batch_transform=True,
@@ -271,6 +271,8 @@ def _execute_new(
             the maximum number of derivatives to support. Increasing this value allows
             for higher order derivatives to be extracted, at the cost of additional
             (classical) computational overhead during the backwards pass.
+        override_shots (int): The number of shots to use for the execution. If ``False``, then the
+            number of shots on the device is used.
         expand_fn (function): Tape expansion function to be called prior to device execution.
             Must have signature of the form ``expand_fn(tape, max_expansion)``, and return a
             single :class:`~.QuantumTape`. If not provided, by default :meth:`Device.expand_fn`
@@ -351,7 +353,8 @@ def _execute_new(
     gradient_kwargs = gradient_kwargs or {}
 
     if device_batch_transform:
-        tapes, batch_fn = qml.transforms.map_batch_transform(device.batch_transform, tapes)
+        dev_batch_transform = set_shots(device, override_shots)(device.batch_transform)
+        tapes, batch_fn = qml.transforms.map_batch_transform(dev_batch_transform, tapes)
     else:
         batch_fn = lambda res: res  # pragma: no cover
 
@@ -481,7 +484,7 @@ def execute(
     cache=True,
     cachesize=10000,
     max_diff=1,
-    override_shots=False,
+    override_shots: int = False,
     expand_fn="device",
     max_expansion=10,
     device_batch_transform=True,
@@ -513,6 +516,8 @@ def execute(
             the maximum number of derivatives to support. Increasing this value allows
             for higher order derivatives to be extracted, at the cost of additional
             (classical) computational overhead during the backwards pass.
+        override_shots (int): The number of shots to use for the execution. If ``False``, then the
+            number of shots on the device is used.
         expand_fn (function): Tape expansion function to be called prior to device execution.
             Must have signature of the form ``expand_fn(tape, max_expansion)``, and return a
             single :class:`~.QuantumTape`. If not provided, by default :meth:`Device.expand_fn`
@@ -610,7 +615,8 @@ def execute(
     gradient_kwargs = gradient_kwargs or {}
 
     if device_batch_transform:
-        tapes, batch_fn = qml.transforms.map_batch_transform(device.batch_transform, tapes)
+        dev_batch_transform = set_shots(device, override_shots)(device.batch_transform)
+        tapes, batch_fn = qml.transforms.map_batch_transform(dev_batch_transform, tapes)
     else:
         batch_fn = lambda res: res
 


Test patch:
diff --git a/tests/interfaces/test_autograd.py b/tests/interfaces/test_autograd.py
index 34f34e2b5..50c6c2d6b 100644
--- a/tests/interfaces/test_autograd.py
+++ b/tests/interfaces/test_autograd.py
@@ -210,6 +210,15 @@ class TestBatchTransformExecution:
         spy.assert_called()
         assert np.allclose(res[0], np.cos(y), atol=0.1)
 
+    def test_batch_transform_dynamic_shots(self):
+        """Tests that the batch transform considers the number of shots for the execution, not those
+        statically on the device."""
+        dev = qml.device("default.qubit", wires=1)
+        H = 2.0 * qml.PauliZ(0)
+        qscript = qml.tape.QuantumScript(measurements=[qml.expval(H)])
+        res = qml.execute([qscript], dev, interface=None, override_shots=10)
+        assert res == [2.0]
+
 
 class TestCaching:
     """Test for caching behaviour"""
diff --git a/tests/returntypes/test_autograd_new.py b/tests/returntypes/test_autograd_new.py
index a910ccafd..a496d72ae 100644
--- a/tests/returntypes/test_autograd_new.py
+++ b/tests/returntypes/test_autograd_new.py
@@ -205,6 +205,15 @@ class TestBatchTransformExecution:
         assert isinstance(res[0], float)
         assert np.allclose(res[0], np.cos(y), atol=0.1)
 
+    def test_batch_transform_dynamic_shots(self):
+        """Tests that the batch transform considers the number of shots for the execution, not those
+        statically on the device."""
+        dev = qml.device("default.qubit", wires=1)
+        H = 2.0 * qml.PauliZ(0)
+        qscript = qml.tape.QuantumScript(measurements=[qml.expval(H)])
+        res = qml.execute([qscript], dev, interface=None, override_shots=10)
+        assert res == [2.0]
+
 
 class TestCaching:
     """Test for caching behaviour"""