message: Fix matmul size bound
  Test Plan: `//tensorflow/compiler/plugin/poplar:matmul_size_test` passes
  Reviewers: #tensorflow, georgep
diff: @@ -84,7 +84,7 @@ class MatMulSizeTest(test_util.TensorFlowTestCase):
  evts = ipu.utils.extract_all_events(out)
  size = ipu.utils.get_memory_size_from_events(evts)
  - self.assertTrue(size < 17740000)
  + self.assertTrue(size < 18780000)
  def testTrainingBs1(self):
  x = array_ops.placeholder(datatype, shape=[1, 112 * 112 * 4])
  @@ -156,7 +156,7 @@ class MatMulSizeTest(test_util.TensorFlowTestCase):
  evts = ipu.utils.extract_all_events(out)
  size = ipu.utils.get_memory_size_from_events(evts)
  - self.assertTrue(size < 26400000)
  + self.assertTrue(size < 38580000)
  if __name__ == "__main__":
  