Add a tensor the watch store.

def add(self, value):
    """Add a tensor the watch store."""
    if self._disposed:
      raise ValueError(
          'Cannot add value: this _WatchStore instance is already disposed')
    self._data.append(value)
    if hasattr(value, 'nbytes'):
      self._in_mem_bytes += value.nbytes
      self._ensure_bytes_limits()