message: Make collective op's communication_hint arg case insensitive.
diff: @@ -60,7 +60,7 @@ def all_reduce(t, group_size, group_key, instance_key, merge_op, final_op,
  merge_op=merge_op,
  final_op=final_op,
  subdiv_offsets=subdiv_offsets,
  - communication_hint=communication_hint)
  + communication_hint=communication_hint.lower())
  def all_gather(t, group_size, group_key, instance_key,
  @@ -93,7 +93,7 @@ def all_gather(t, group_size, group_key, instance_key,
  group_size=group_size,
  group_key=group_key,
  instance_key=instance_key,
  - communication_hint=communication_hint)
  + communication_hint=communication_hint.lower())
  def broadcast_send(t, shape, dtype, group_size, group_key, instance_key,
  @@ -146,7 +146,7 @@ def broadcast_send(t, shape, dtype, group_size, group_key, instance_key,
  group_size=group_size,
  group_key=group_key,
  instance_key=instance_key,
  - communication_hint=communication_hint)
  + communication_hint=communication_hint.lower())
  def broadcast_recv(shape, dtype, group_size, group_key, instance_key,
  @@ -180,4 +180,4 @@ def broadcast_recv(shape, dtype, group_size, group_key, instance_key,
  group_size=group_size,
  group_key=group_key,
  instance_key=instance_key,
  - communication_hint=communication_hint)
  + communication_hint=communication_hint.lower())
  