small fixes

Author: renatomello

src/qibo/backends/abstract.py

@@ -315,7 +315,7 @@ def add_at(
             indices (ArrayLike): Indices at which to add elements.
             array_2 (ArrayLike): Input array with elements to add.
         """
-        return self.engine.add.at(array_1, indices, array_2)
+        self.engine.add.at(array_1, indices, array_2)
 
     def all(self, array: ArrayLike, **kwargs) -> Union[bool, ArrayLike]:
         """Test whether all ``array`` elements evaluate to ``True``.
@@ -1012,6 +1012,8 @@ def logm(self, array: ArrayLike, **kwargs) -> ArrayLike:  # pragma: no cover
 
         Args:
             array (ArrayLike): input array.
+            kwargs (optional): additional options for this function.
+                For more details, see the corresponding engine's documentation.
 
         Returns:
             ArrayLike: The resulting matrix logarithm.

src/qibo/backends/numpy.py

@@ -122,6 +122,18 @@ def block_diag(self, *arrays: ArrayLike) -> ArrayLike:
         return block_diag(*arrays)
 
     def coo_matrix(self, array: ArrayLike, **kwargs) -> ArrayLike:  # pragma: no cover
+        """Return the sparse version of ``array`` in coordinate format.
+
+        Also known as the ``ijv`` or ``triplet`` format.
+
+        Args:
+            array (ArrayLike): input array.
+            kwargs (optional): additional options for this function.
+                For more details, see the corresponding engine's documentation.
+
+        Returns:
+            ArrayLike: The coordinate-format version of ``array``.
+        """
         return coo_matrix(array, **kwargs)
 
     def csr_matrix(self, array: ArrayLike, **kwargs) -> ArrayLike: