eager: Always run dataset iterator operations on CPU.

It has no kernels for other devices.
With an explicit "tf.device()" before invoking the kernel we ensure
that Iterator.next() functions even when placed inside a:

with tf.device("/device:GPU:0")

PiperOrigin-RevId: 171048558

tensorflow/contrib/eager/python/datasets.py

@@ -23,6 +23,7 @@ import threading
 from tensorflow.python.data.util import nest
 from tensorflow.python.eager import context
 from tensorflow.python.framework import errors
+from tensorflow.python.framework import ops
 from tensorflow.python.ops import gen_dataset_ops
 from tensorflow.python.ops import resource_variable_ops
 
@@ -62,6 +63,7 @@ class Iterator(object):
       raise RuntimeError(
           "{} objects only make sense when eager execution is enabled".format(
               type(self)))
+    with ops.device("/device:CPU:0"):
       ds_variant = dataset._as_variant_tensor()  # pylint: disable=protected-access
       self._output_types = dataset.output_types
       self._flat_output_types = nest.flatten(dataset.output_types)
@@ -75,6 +77,7 @@ class Iterator(object):
 
   def __del__(self):
     if self._resource is not None:
+      with ops.device("/device:CPU:0"):
         resource_variable_ops.destroy_resource_op(self._resource)
     self._resource = None
 
@@ -87,6 +90,10 @@ class Iterator(object):
   def next(self):
     """Return the next tf.Tensor from the dataset."""
     try:
+      # TODO(ashankar): Consider removing this ops.device() contextmanager
+      # and instead mimic ops placement in graphs: Operations on resource
+      # handles execute on the same device as where the resource is placed.
+      with ops.device("/device:CPU:0"):
         ret = gen_dataset_ops.iterator_get_next(
             self._resource,
             output_types=self._flat_output_types,