Add reduction_unroll_factor to autotuning script

[rdspring1]

This PR renames unroll_factor to iteration_unroll_factor and adds reduction_unroll_factor. reduction_unroll_factor adds unroll factor on top of vectorization factor for the inner reduction domain.

[liqiangxl]

Should be ceil_div(ceil_div(num_reductions/vectorize_factor, bdimx), reduction_unroll_factor)

[liqiangxl]

This looks strange to me. Why grdim = remaining_reduction? We can do serial reduction instread of grid reduction.

[liqiangxl]

nvFuser's default heuristic does:

  // When iteration dim is small, may have unused SMs, to increase SM usage
  // needs to shift from block reduction to grid reduction.
  int64_t grdim = 1;
  while (godim * grdim * 2 <= sm_count && getInnerRemainder() / grdim >= 2) {
    grdim *= 2;
  }

[rdspring1]

From inner2dReductionHeuristic, I see this:

  // Cross grid reduction if we haven't hit our target blocks, and we have manyr
  // reduction elements.
  if ((godim < target_blocks && remainder_in_reduction >= 0) ||
      (remainder_in_reduction >= kEight)) {
    grdim = remainder_in_reduction;
  }

   // Try to do some cleanup of ragged waves on device
   { do_something }

   // Grid reductions do not support unrolling iteration dimension, revert if
   // set. Recalculate godim.
   { do_something }

[liqiangxl]

another approch is we can add another search para is_block_reduction, if it is true, we only use block reduction, if it is false, we do grid reduction.

[liqiangxl]

LGTM.

[rdspring1]

!build