message: [XLA] Clarify that init_value of Reduce needs to be an identity of the reduction.
  NFC
diff: @@ -1914,9 +1914,10 @@ The output of the op is `Collate(Q_0, ..., Q_N)` where `Q_i` is an array of type
  This operation reduces one or more dimensions of each input array into scalars.
  The rank of each returned array is `rank(operand) - len(dimensions)`. The
  initial value used for every reduction is `init_value`, and it may be inserted
  -anywhere during computation by the back-end. In most cases, `init_value` is an
  -identity of the reduction function (for example, `0` for addition). The applied
  -`computation` is always passed the `init_value` on the left-hand side.
  +anywhere during computation by the back-end. It is required that `init_value` is
  +an identity of the reduction function (for example, `0` for addition) or
  +undefined behavior will occur. The applied `computation` is always passed the
  +`init_value` on the left-hand side.
  The evaluation order of the reduction function is arbitrary and may be
  non-deterministic. Therefore, the reduction function should not be overly
  