add pt2tf tool

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

@@ -0,0 +1,20 @@
+import numpy as np
+import tensorflow as tf
+
+from onnx_tf.handlers.backend_handler import BackendHandler
+from onnx_tf.handlers.handler import onnx_op
+from onnx_tf.common import data_type
+from onnx import mapping
+
+
+@onnx_op("SequenceEmpty")
+class SequenceEmpty(BackendHandler):
+
+  @classmethod
+  def version_11(cls, node, **kwargs):
+    default_dtype = mapping.NP_TYPE_TO_TENSOR_TYPE[np.dtype('float32')]
+    dtype = data_type.onnx2tf(node.attrs.get("dtype", default_dtype))
+
+    ragged = tf.RaggedTensor.from_row_lengths(values=[], row_lengths=[])
+    sparse = tf.cast(ragged.to_sparse(), dtype)
+    return [tf.RaggedTensor.from_sparse(sparse)]