fix issue #2091 (#2107)

* fix EnasMutator LSTMCell forward bug

fix issue #2091

* add comment
Co-authored-by: QuanluZhang <z.quanluzhang@gmail.com>

src/sdk/pynni/nni/nas/pytorch/enas/mutator.py

@@ -23,7 +23,9 @@ class StackedLSTMCell(nn.Module):
             curr_c, curr_h = m(inputs, (prev_c[i], prev_h[i]))
             next_c.append(curr_c)
             next_h.append(curr_h)
-            inputs = curr_h[-1]
+            # current implementation only supports batch size equals 1,
+            # but the algorithm does not necessarily have this limitation
+            inputs = curr_h[-1].view(1, -1)
         return next_c, next_h