message: Change remote buffer merging default in tests
  Summary: Ref. T37049.
  Reviewers: #tensorflow, simonl, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, markf, georgep
  Maniphest Tasks: T37049
diff: @@ -152,7 +152,7 @@ class PipelineTester(object):
  device_mapping,
  number_of_io_tiles=0,
  replication_factor=1,
  - merge_remote_buffers=MergeRemoteBuffersBehaviour.NO_MERGING,
  + merge_remote_buffers=MergeRemoteBuffersBehaviour.IF_BENEFICIAL,
  replicated_optimizer_state_sharding=False,
  minimum_remote_tensor_size=128,
  return_vars=False):
  @@ -262,7 +262,7 @@ class PipelineTester(object):
  return_report=False,
  replication_factor=1,
  offload_activations=None,
  - merge_remote_buffers=MergeRemoteBuffersBehaviour.NO_MERGING,
  + merge_remote_buffers=MergeRemoteBuffersBehaviour.IF_BENEFICIAL,
  replicated_optimizer_state_sharding=False,
  minimum_remote_tensor_size=128,
  return_vars=False,
  @@ -447,7 +447,7 @@ class PipelineTester(object):
  recomputation_mode=None,
  number_of_io_tiles=0,
  offload_activations=None,
  - merge_remote_buffers=MergeRemoteBuffersBehaviour.NO_MERGING,
  + merge_remote_buffers=MergeRemoteBuffersBehaviour.IF_BENEFICIAL,
  replication_factor=1,
  replicated_optimizer_state_sharding=False,
  minimum_remote_tensor_size=128):
  