message: BFCAllocator: lower log level for empty allocations/deallocations.
  It's not generally an error to allocate/deallocate 0 bytes. I ran into
  this experimenting with using BFCAllocator in jax, and was getting
  lots of error log spew.
diff: @@ -262,7 +262,7 @@ void* BFCAllocator::AllocateRawInternal(size_t unused_alignment,
  bool dump_log_on_failure,
  uint64 freed_before) {
  if (num_bytes == 0) {
  - LOG(ERROR) << "tried to allocate 0 bytes";
  + VLOG(2) << "tried to allocate 0 bytes";
  return nullptr;
  }
  // First, always allocate memory of at least kMinAllocationSize
  @@ -405,7 +405,7 @@ void BFCAllocator::DeallocateRaw(void* ptr) {
  void BFCAllocator::DeallocateRawInternal(void* ptr) {
  if (ptr == nullptr) {
  - LOG(ERROR) << "tried to deallocate nullptr";
  + VLOG(2) << "tried to deallocate nullptr";
  return;
  }
  mutex_lock l(lock_);
  