message: Move pipeline op creation into a Session context
diff: @@ -150,6 +150,8 @@ class PipeliningGroupedTest(test_util.TensorFlowTestCase):
  with ops.device('cpu'):
  c = array_ops.placeholder(np.float32, shape=[])
  + with tu.ipu_session() as sess:
  +
  with ops.device("/device:IPU:0"):
  r = ipu_compiler.compile(my_net, inputs=[c])
  @@ -158,7 +160,6 @@ class PipeliningGroupedTest(test_util.TensorFlowTestCase):
  utils.configure_ipu_system(cfg)
  utils.move_variable_initialization_to_cpu()
  - with tu.ipu_session() as sess:
  sess.run(variables.global_variables_initializer())
  sess.run(infeed_queue.initializer)
  with self.assertRaisesRegex(
  @@ -277,6 +278,8 @@ class PipeliningGroupedTest(test_util.TensorFlowTestCase):
  with ops.device('cpu'):
  c = array_ops.placeholder(np.float32, shape=[])
  + with tu.ipu_session() as sess:
  +
  with ops.device("/device:IPU:0"):
  r = ipu_compiler.compile(my_net, inputs=[c])
  @@ -286,7 +289,7 @@ class PipeliningGroupedTest(test_util.TensorFlowTestCase):
  utils.move_variable_initialization_to_cpu()
  outfeed_op = outfeed_queue.dequeue()
  - with tu.ipu_session() as sess:
  +
  report = tu.ReportJSON(self, sess, configure_device=False)
  sess.run(variables.global_variables_initializer())
  sess.run(infeed_queue.initializer)
  @@ -344,6 +347,8 @@ class PipeliningGroupedTest(test_util.TensorFlowTestCase):
  with ops.device('cpu'):
  c = array_ops.placeholder(np.float32, shape=[])
  + with tu.ipu_session() as sess:
  +
  with ops.device("/device:IPU:0"):
  r = ipu_compiler.compile(my_net, inputs=[c])
  @@ -353,7 +358,7 @@ class PipeliningGroupedTest(test_util.TensorFlowTestCase):
  utils.move_variable_initialization_to_cpu()
  outfeed_op = outfeed_queue.dequeue()
  - with tu.ipu_session() as sess:
  +
  report = tu.ReportJSON(self, sess, configure_device=False)
  report.reset()
  sess.run(variables.global_variables_initializer())
  @@ -408,6 +413,8 @@ class PipeliningGroupedTest(test_util.TensorFlowTestCase):
  with ops.device('cpu'):
  c = array_ops.placeholder(np.float32, shape=[])
  + with tu.ipu_session() as sess:
  +
  with ops.device("/device:IPU:0"):
  r = ipu_compiler.compile(my_net, inputs=[c])
  @@ -417,7 +424,7 @@ class PipeliningGroupedTest(test_util.TensorFlowTestCase):
  utils.move_variable_initialization_to_cpu()
  outfeed_op = outfeed_queue.dequeue()
  - with tu.ipu_session() as sess:
  +
  report = tu.ReportJSON(self, sess, configure_device=False)
  sess.run(variables.global_variables_initializer())
  sess.run(infeed_queue.initializer)
  