[nn] implement extend method to sequential class (#81179)

Follows #71329

cc @kshitij12345 :)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81179
Approved by: https://github.com/albanD

test/test_nn.py

@@ -1600,6 +1600,19 @@ class TestNN(NNTestCase):
         self.assertEqual(n2, nn.Sequential(l1, l2, l3, l4))
         self.assertEqual(nn.Sequential(l1).append(l2).append(l4), nn.Sequential(l1, l2, l4))
 
+    def test_Sequential_extend(self):
+        l1 = nn.Linear(10, 20)
+        l2 = nn.Linear(20, 30)
+        l3 = nn.Linear(30, 40)
+        l4 = nn.Linear(40, 50)
+        n1 = nn.Sequential(l1, l2)
+        n2 = nn.Sequential(l3, l4)
+        n3 = nn.Sequential(l1, l2)
+        for l in n2:
+            n1.append(l)
+        n3.extend(n2)
+        self.assertEqual(n3, n1)
+
     def test_ModuleList(self):
         modules = [nn.ReLU(), nn.Linear(5, 5)]
         module_list = nn.ModuleList(modules)

torch/nn/modules/container.py

@@ -163,6 +163,11 @@ class Sequential(Module):
         self.add_module(str(len(self)), module)
         return self
 
+    def extend(self, sequential) -> 'Sequential':
+        for layer in sequential:
+            self.append(layer)
+        return self
+
 
 class ModuleList(Module):
     r"""Holds submodules in a list.