[nn] add + operator for torch.nn.Sequential to concatenate (#81170)

Fixes #78512

#### TODO
- [x] add tests

cc @kshitij12345!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81170
Approved by: https://github.com/albanD

test/test_nn.py

@@ -1580,6 +1580,15 @@ class TestNN(NNTestCase):
         del n[1::2]
         self.assertEqual(n, nn.Sequential(l1, l3))
 
+    def test_Sequential_add(self):
+        l1 = nn.Linear(1, 2)
+        l2 = nn.Linear(2, 3)
+        l3 = nn.Linear(3, 4)
+        l4 = nn.Linear(4, 5)
+        n = nn.Sequential(l1, l2)
+        other = nn.Sequential(l3, l4)
+        self.assertEqual(n + other, nn.Sequential(l1, l2, l3, l4))
+
     def test_Sequential_append(self):
         l1 = nn.Linear(10, 20)
         l2 = nn.Linear(20, 30)

torch/nn/modules/container.py

@@ -122,6 +122,19 @@ class Sequential(Module):
     def __len__(self) -> int:
         return len(self._modules)
 
+    def __add__(self, other) -> 'Sequential':
+        if isinstance(other, Sequential):
+            ret = Sequential()
+            for layer in self:
+                ret.append(layer)
+            for layer in other:
+                ret.append(layer)
+            return ret
+        else:
+            raise ValueError('add operator supports only objects '
+                             'of Sequential class, but {} is given.'.format(
+                                 str(type(other))))
+
     @_copy_to_script_wrapper
     def __dir__(self):
         keys = super(Sequential, self).__dir__()