Remove warning example qsvt (#6693)

**Context:**

- The example shows a warning due to the deprecation of qml.qsvt_legacy.
It has been updated
- This PR also add a little modification to accept scalars and arrays as
qsvt input ([solves this
comment](PennyLaneAI/qml#1276 (comment)))

---------

Co-authored-by: Jay Soni <[email protected]>

doc/releases/changelog-dev.md

@@ -247,6 +247,7 @@ such as `shots`, `rng` and `prng_key`.
 * The `qml.qsvt` function has been improved to be more user-friendly. Old functionality is moved to `qml.qsvt_legacy`
   and it will be deprecated in release v0.40.
   [(#6520)](https://github.com/PennyLaneAI/pennylane/pull/6520/)
+  [(#6693)](https://github.com/PennyLaneAI/pennylane/pull/6693)
 
 <h4>Other Improvements</h4>
 

pennylane/templates/subroutines/qsvt.py

@@ -229,7 +229,7 @@ def qsvt(A, poly, encoding_wires=None, block_encoding=None, **kwargs):
 
         @qml.qnode(dev)
         def circuit():
-            qml.qsvt(hamiltonian, poly, encoding_wires=[0])
+            qml.qsvt(hamiltonian, poly, encoding_wires=[0], block_encoding="prepselprep")
             return qml.state()
 
 
@@ -328,9 +328,9 @@ def circuit():
 
     if encoding_wires is None or block_encoding is None or "wires" in kwargs.keys():
         warnings.warn(
-            "You may be trying to use the old `qsvt` functionality (now `qml.qsvt_legacy`)."
-            "Make sure you pass a polynomial instead of angles."
-            "Set a value for `block_encoding` to silence this warning.",
+            "You may be trying to use the old `qsvt` functionality (now `qml.qsvt_legacy`).\n"
+            "Make sure you pass a polynomial instead of angles.\n"
+            "Set a value for `block_encoding` to silence this warning.\n",
             qml.PennyLaneDeprecationWarning,
         )
 
@@ -369,7 +369,7 @@ def circuit():
                 "block_encoding = {block_encoding} not supported for A of type {type(A)}. When A is a matrix block_encoding should take the value 'embedding' or 'fable'. Otherwise, please provide an input with a Pauli decomposition. For more details, see the 'qml.pauli_decompose' function."
             )
 
-        A = qml.math.array(A)
+        A = qml.math.atleast_2d(A)
         max_dimension = 1 if len(qml.math.array(A).shape) == 0 else max(A.shape)
 
         if block_encoding == "fable":

tests/templates/test_subroutines/test_qsvt.py

@@ -696,6 +696,12 @@ def test_qsvt_warning(self):
                 "fable",
                 [0, 1, 2, 3, 4],
             ),
+            (
+                0.3,
+                [0.2, 0, 0.3],
+                "embedding",
+                [0],
+            ),
         ],
     )
     def test_matrix_input(self, A, poly, encoding_wires, block_encoding):
@@ -707,10 +713,11 @@ def circuit():
             qml.qsvt(A, poly, encoding_wires, block_encoding)
             return qml.state()
 
+        A_matrix = qml.math.atleast_2d(A)
         # Calculation of the polynomial transformation on the input matrix
-        expected = sum(coef * matrix_power(A, i) for i, coef in enumerate(poly))
+        expected = sum(coef * matrix_power(A_matrix, i) for i, coef in enumerate(poly))
 
-        assert np.allclose(qml.matrix(circuit)()[: len(A), : len(A)].real, expected)
+        assert np.allclose(qml.matrix(circuit)()[: len(A_matrix), : len(A_matrix)].real, expected)
 
     @pytest.mark.parametrize(
         ("A", "poly", "block_encoding", "encoding_wires"),