message: [XLA] Update doc for collective permute.
diff: @@ -418,6 +418,33 @@ then v12 == f32[8x3] {{10, 11, 12},
  ```
  +## CollectivePermute
  +
  +See also
  +[`XlaBuilder::CollectivePermute`](https://www.tensorflow.org/code/tensorflow/compiler/xla/client/xla_builder.h).
  +
  +CollectivePermute is a collective operation that sends and receives data cross
  +replicas.
  +
  +<b> `CollectivePermute(operand, source_target_pairs)` </b>
  +
  +| Arguments | Type | Semantics |
  +| --------------------- | ----------------------- | -------------------------- |
  +| `operand` | `XlaOp` | n dimensional input array |
  +| `source_target_pairs` | `<int64, int64>` vector | A list of |
  +: : : (source_replica_id, :
  +: : : target_replica_id) pairs. :
  +: : : For each pair, the operand :
  +: : : is sent from source :
  +: : : replica to target replica. :
  +
  +Note that there are the following restrictions on the `source_target_pair`:
  +
  +- Any two pairs should not have the same target replica id, and they should
  + not have the same source replica id.
  +- If a replica id is not a target in any pair, then the output on that replica
  + is a tensor consists of 0(s) with the same shape as the input.
  +
  ## Concatenate
  See also
  