feat(frontend-python): implement len for tracers

This makes fhe.array extension work with tensors as well!

frontends/concrete-python/concrete/fhe/extensions/array.py

@@ -44,9 +44,7 @@ def array(values: Any) -> Union[np.ndarray, Tracer]:
         if not isinstance(value, Tracer):
             values[i] = Tracer.sanitize(value)
 
-        if not values[i].output.is_scalar:
-            message = "Encrypted arrays can only be created from scalars"
-            raise ValueError(message)
+        assert values[i].output.is_scalar
 
     dtype = combine_dtypes([value.output.dtype for value in values])
     is_encrypted = True

frontends/concrete-python/concrete/fhe/tracing/tracer.py

@@ -863,6 +863,13 @@ class Tracer:
 
         return Tracer._trace_numpy_operation(np.transpose, self)
 
+    def __len__(self):
+        shape = self.shape
+        if len(shape) == 0:
+            message = "object of type 'Tracer' where 'shape == ()' has no len()"
+            raise TypeError(message)
+        return shape[0]
+
 
 class Annotation(Tracer):
     """