Remove multiprocessing as well

The .nthreads attribute is then set to 0, to avoid the default coming from the abstract (currently 1), since TensorFlow will use multiple threads by default

src/qibo/backends/tensorflow.py

@@ -1,5 +1,4 @@
 import collections
-import multiprocessing
 import os
 
 import numpy as np
@@ -196,7 +195,7 @@ def __init__(self):
         elif cpu_devices:
             self.device = cpu_devices[0].name
 
-        self.nthreads = multiprocessing.cpu_count()
+        self.nthreads = 0
 
         self.tensor_types = (np.ndarray, tf.Tensor, tf.Variable)