Symbolic representation for unfold using ATen (pytorch#3334)

Author: James Reed

Diff for: torch/onnx/symbolic.py

@@ -233,3 +233,7 @@ def avg_pool2d(g, input, kernel_size, stride, padding, ceil_mode, count_include_
 
 def log_softmax(g, input, dim=None):
     return g.op("Log", g.op('Softmax', input, axis_i=dim).setTypeAs(input))
+
+
+def unfold(g, input, dimension, size, step):
+    return g.op("ATen", input, operator_s="unfold", dimension_i=dimension, size_i=size, step_i=step)