add pt2tf tool

pt2tf/onnx-tensorflow/onnx_tf/handlers/backend/non_zero.py

@@ -0,0 +1,15 @@
+import tensorflow as tf
+
+from onnx_tf.handlers.backend_handler import BackendHandler
+from onnx_tf.handlers.handler import onnx_op
+
+
+@onnx_op("NonZero")
+class NonZero(BackendHandler):
+
+  @classmethod
+  def version_9(cls, node, **kwargs):
+    input_tensor = kwargs["tensor_dict"][node.inputs[0]]
+    condition = tf.not_equal(input_tensor, tf.zeros_like(input_tensor))
+    nonzero_indices = tf.where(condition)
+    return [tf.transpose(nonzero_indices)]