message: Change fragmentation metric computed by BFCAllocator to a simpler
  algorithm, i.e. the inverse ratio of largest free block to total free bytes.
diff: @@ -1006,7 +1006,7 @@ MemoryDump BFCAllocator::RecordMemoryMapInternal() {
  mas->set_bytes_in_use(stats_.bytes_in_use);
  mas->set_peak_bytes_in_use(stats_.peak_bytes_in_use);
  mas->set_largest_alloc_size(stats_.largest_alloc_size);
  - double frag_m_sum = 0.0;
  + int64 largest_free_chunk = 0;
  int64 free_bytes = 0;
  // Record summary data for every bin.
  @@ -1045,13 +1045,19 @@ MemoryDump BFCAllocator::RecordMemoryMapInternal() {
  }
  if (!c->in_use()) {
  free_bytes += c->size;
  - frag_m_sum += pow(c->size, 1.1);
  + if (c->size > largest_free_chunk) {
  + largest_free_chunk = c->size;
  + }
  }
  h = c->next;
  }
  }
  - mas->set_fragmentation_metric(
  - 1.0 - (frag_m_sum / pow(static_cast<double>(free_bytes), 1.1)));
  + double frag_metric = 0.0;
  + if (free_bytes > 0) {
  + frag_metric =
  + (free_bytes - largest_free_chunk) / static_cast<double>(free_bytes);
  + }
  + mas->set_fragmentation_metric(frag_metric);
  #ifdef TENSORFLOW_MEM_DEBUG
  // Record the recent size history
  