Add `init` member to ReduceNode (#6138)

Add `init` member to ReduceNode (#6138)

- This patch adds a new member to ReduceNode called init which allows
  initialization with a custom ProducerLoad or a Float/Int immediate.
- This allows initialization of the output Tensor of a reduction with
  another Tensor instead of the `identity_element` defined in the
  CommReducer
- One example use case for this node is to initialize the Output of a
  convolution reduction with the Bias values thereby saving the
  Bias-add computation.