lib/tensor_stream/utils.rb
def placeholder(dtype, shape: nil, name: nil)
      TensorStream::Placeholder.new(dtype, nil, shape, name: name)
    end
def placeholder(dtype, shape: nil, name: nil)
      TensorStream::Placeholder.new(dtype, nil, shape, name: name)
    end
Inserts a placeholder for a tensor that will be always fed.