message: Add '/Zero' to the compute set list, part of the new slice update
  Summary: As title.  This new compute set is part of the slice update.
  Test Plan: All tests ok
  Reviewers: #tensorflow, georgep
  Subscribers: georgep
diff: @@ -720,6 +720,7 @@ class IpuFuseOpsTest(xla_test.XLATestCase):
  ok = [
  '__seed*',
  + '/Zero',
  'ExpandDims/input/fusion*/multiUpdateAdd',
  'ExpandDims/input/fusion*/NegateScale/Op/Negate',
  'vs/Gather*/gather.*/multiSlice',
  @@ -782,6 +783,7 @@ class IpuFuseOpsTest(xla_test.XLATestCase):
  ok = [
  '__seed*',
  + '/Zero',
  'ExpandDims/input/fusion*/multiUpdateAdd',
  'ExpandDims/input/fusion*/NegateScale/Op/Negate',
  'vs/Gather*/gather.*/multiSlice',
  @@ -836,6 +838,7 @@ class IpuFuseOpsTest(xla_test.XLATestCase):
  # pylint: disable=line-too-long
  ok = [
  '__seed*',
  + '/Zero',
  'GradientDescent/update_vs/w/Neg/negate*/Op/Negate',
  'GradientDescent/update_vs/w/mul/fusion*/Op/Multiply',
  'GradientDescent/update_vs/w/ResourceScatterAdd/custom-call*/multiUpdateAdd',
  @@ -901,6 +904,7 @@ class IpuFuseOpsTest(xla_test.XLATestCase):
  # pylint: disable=line-too-long
  ok = [
  '__seed*',
  + '/Zero',
  'GradientDescent/update_vs/w/ResourceApplyGradientDescent/fusion*/multiUpdateAdd',
  'GradientDescent/update_vs/w/ResourceApplyGradientDescent/fusion*/NegateScale/Op/Negate',
  'gradients/vs/absolute_difference/Abs_grad/Sign',
  