Add CPU and GPU into backends

src/qibotn/backends/gpu.py

@@ -0,0 +1,39 @@
+from qibo.backends.numpy import NumpyBackend
+
+
+class CuTensorNet(NumpyBackend):  # pragma: no cover
+    # CI does not test for GPU
+
+    def __init__(self):
+        super().__init__()
+        import cuquantum  # pylint: disable=import-error
+        from cuquantum import cutensornet as cutn  # pylint: disable=import-error
+
+        self.cuquantum = cuquantum
+        self.cutn = cutn
+        self.platform = "cutensornet"
+        self.versions["cuquantum"] = self.cuquantum.__version__
+        self.supports_multigpu = True
+        self.handle = self.cutn.create()
+
+    def __del__(self):
+        if hasattr(self, "cutn"):
+            self.cutn.destroy(self.handle)
+
+    def set_precision(self, precision):
+        if precision != self.precision:
+            super().set_precision(precision)
+
+    def get_cuda_type(self, dtype="complex64"):
+        if dtype == "complex128":
+            return (
+                self.cuquantum.cudaDataType.CUDA_C_64F,
+                self.cuquantum.ComputeType.COMPUTE_64F,
+            )
+        elif dtype == "complex64":
+            return (
+                self.cuquantum.cudaDataType.CUDA_C_32F,
+                self.cuquantum.ComputeType.COMPUTE_32F,
+            )
+        else:
+            raise TypeError("Type can be either complex64 or complex128")