remove unused class (#3123)

### Changes

Removed TensorBackendType.
### Reason for changes

Got TensorBackendType and TensorBackend with the same definition

nncf/tensor/__init__.py

@@ -9,7 +9,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from nncf.tensor.definitions import TensorBackendType as TensorBackendType
+from nncf.tensor.definitions import TensorBackend as TensorBackend
 from nncf.tensor.definitions import TensorDataType as TensorDataType
 from nncf.tensor.definitions import TensorDeviceType as TensorDeviceType
 from nncf.tensor.tensor import Tensor as Tensor

nncf/tensor/definitions.py

@@ -14,13 +14,13 @@
 from enum import auto
 
 
-class TensorBackendType(Enum):
+class TensorBackend(Enum):
     """
     Enum representing the different tensor backends.
     """
 
-    NUMPY = auto()
-    TORCH = auto()
+    numpy = auto()
+    torch = auto()
 
 
 class TensorDataType(Enum):
@@ -53,15 +53,6 @@ class TensorDeviceType(Enum):
     GPU = auto()
 
 
-class TensorBackend(Enum):
-    """
-    Enum representing the different tensor backends.
-    """
-
-    numpy = auto()
-    torch = auto()
-
-
 @dataclass
 class TypeInfo:
     """