add pt2tf tool

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

@@ -0,0 +1,26 @@
+import tensorflow as tf
+
+from onnx_tf.handlers.backend_handler import BackendHandler
+from onnx_tf.handlers.handler import onnx_op
+
+
+@onnx_op("SequenceConstruct")
+class SequenceConstruct(BackendHandler):
+
+  @classmethod
+  def version_11(cls, node, **kwargs):
+    # create an empty sequence first
+    tensor_dict = kwargs["tensor_dict"]
+    dtype = tensor_dict[node.inputs[0]].dtype
+    input_sequence = tf.ragged.constant([], dtype=dtype)
+
+    # insert tensors at the end of sequence
+    for i in range(len(node.inputs)):
+      input_tensor = tf.expand_dims(tensor_dict[node.inputs[i]], 0)
+      if input_sequence.shape[0] == 0:
+        output_seq = tf.RaggedTensor.from_tensor(input_tensor)
+      else:
+        output_seq = tf.concat([input_sequence, input_tensor], axis=0)
+      input_sequence = output_seq
+
+    return [output_seq]