Add parser support for CAST tflite operator

This implementation provides cast to limited number of dtypes that tflite
currently supports for placeholder op. The encoding values for
placeholder dtypes are taken from the tflite flatbuff schema.

python/tvm/relay/frontend/tflite.py

@@ -87,6 +87,7 @@ def __init__(self, model, subgraph, exp_tab):
             'RELU':self.convert_relu,
             'SPLIT': self.convert_split,
             'TRANSPOSE': self.convert_transpose,
+            'CAST': self.convert_cast,
             'TILE': self.convert_tile,
             'BATCH_TO_SPACE_ND': self.convert_batch_to_space_nd,
             'SPACE_TO_BATCH_ND': self.convert_space_to_batch_nd
@@ -820,6 +821,38 @@ def convert_transpose(self, op):
 
         return out
 
+    def convert_cast(self, op):
+        """Convert TFLite CAST"""
+        try:
+            from tflite.Operator import Operator
+            from tflite.BuiltinOptions import BuiltinOptions
+            from tflite.CastOptions import CastOptions
+        except ImportError:
+            raise ImportError("The tflite package must be installed")
+
+        assert isinstance(op, Operator)
+        input_tensors = self.get_input_tensors(op)
+        assert len(input_tensors) == 1, "input tensors length should be 1"
+        input_tensor = input_tensors[0]
+        in_expr = self.get_expr(input_tensor.tensor_idx)
+
+        assert op.BuiltinOptionsType() == BuiltinOptions.CastOptions
+        op_options = op.BuiltinOptions()
+        cast_options = CastOptions()
+        cast_options.Init(op_options.Bytes, op_options.Pos)
+        cast_dtype = cast_options.OutDataType()
+
+        # a dict to decode the enumeriton of tensor dtypes as in the tflite fb schema
+        type_decoder = {
+            0: 'float32',
+            2: 'int32',
+            3: 'uint8',
+            4: 'int64'
+        }
+        out = _op.cast(in_expr, type_decoder[cast_dtype])
+
+        return out
+
     def convert_tile(self, op):
         """tile implementation."""
         try:

tests/python/frontend/tflite/test_forward.py

@@ -229,6 +229,24 @@ def test_forward_transpose():
     _test_forward_transpose((2, 3, 4, 5), (3, 0, 1, 2))
     _test_forward_transpose((2, 3, 4, 5), ())
 
+#######################################################################
+# Cast
+# --------
+
+def _test_cast(data, cast_dtype):
+    """ One iteration of CAST """
+    with tf.Graph().as_default():
+        in_data = array_ops.placeholder(shape=data.shape, dtype=data.dtype)
+        out = math_ops.cast(in_data, cast_dtype)
+        compare_tflite_with_tvm(data, 'Placeholder:0', [in_data], [out])
+
+
+def test_forward_cast():
+    """ CAST """
+    _test_cast(np.arange(6.0, dtype=np.float32).reshape((1, 6)), cast_dtype=tf.int32)
+    _test_cast(np.arange(6.0, dtype=np.float32).reshape((1, 6)), cast_dtype=tf.uint8)
+    _test_cast(np.arange(6.0, dtype=np.int64).reshape((1, 6)), cast_dtype=tf.int32)
+
 #######################################################################
 # tile
 # ---------
@@ -997,6 +1015,9 @@ def test_forward_ssd_mobilenet_v1():
     # Transpose
     test_forward_transpose()
 
+    # Cast
+    test_forward_cast()
+
     # Tile
     test_forward_tile()