Remove matmuleinsum backend

src/qibo/models/variational.py

@@ -1,4 +1,3 @@
-from qibo import get_backend
 from qibo.config import raise_error
 from qibo.core.circuit import Circuit
 from qibo.models.evolution import StateEvolution
@@ -76,20 +75,15 @@ def _loss(params, circuit, hamiltonian):
             if K.op is not None:
                 raise_error(RuntimeError, "Cannot compile VQE that uses custom operators. "
                                           "Set the compile flag to False.")
-            from qibo import K
             for gate in self.circuit.queue:
                 _ = gate.cache
             loss = K.compile(_loss)
-
-        if method == 'sgd':
-            # check if gates are using the MatmulEinsum backend
-            if K.op is not None:
-                raise_error(RuntimeError, 'SGD VQE requires native Tensorflow '
-                                          'gates because gradients are not '
-                                          'supported in the custom kernels.')
-            loss = _loss
         else:
+            loss = _loss
+
+        if method != "sgd":
             loss = lambda p, c, h: K.qnp.dtypes("DTYPE")(_loss(p, c, h))
+
         result, parameters, extra = self.optimizers.optimize(loss, initial_state,
                                                              args=(self.circuit, self.hamiltonian),
                                                              method=method, jac=jac, hess=hess, hessp=hessp,

src/qibo/optimizers.py

@@ -164,15 +164,11 @@ def sgd(loss, initial_parameters, args=(), options=None, compile=False):
               - ``'nmessage'`` (int, default: ``1e3``): Every how many epochs to print
                 a message of the loss function.
     """
-    # check if gates are using the MatmulEinsum backend
-    compatible_backends = {
-        "tensorflow_defaulteinsum", "tensorflow_matmuleinsum"}
     from qibo import K
-    if K.name not in compatible_backends:  # pragma: no cover
-        from qibo.config import raise_error
-        raise_error(RuntimeError, "SGD requires native Tensorflow backend.")
+    from qibo.config import log, raise_error
+    if K.name != "tensorflow":
+        raise_error(RuntimeError, "SGD optimizer requires Tensorflow backend.")
 
-    from qibo.config import log
     sgd_options = {"nepochs": 1000000,
                    "nmessage": 1000,
                    "optimizer": "Adagrad",