通过利用 data_flow_ops.StagingArea，每个阶段的主要部分与其他阶段并行执行。StagingArea 是一个像队列（queue）一样且类似于 tf.FIFOQueue 的运算符。不同之处在于 StagingArea 提供了更简单的功能且可在 CPU 和 GPU 中与其他阶段并行执行。将输入管道拆分为 3 个独立并行操作的阶段，并且这是可扩展的，充分利用大型的多核环境。本节的余下部分将详细介绍每个阶段以及 data_flow_ops.StagingArea 的使用细节。
class StagingArea(object):
"""Class for staging inputs. No ordering guarantees.
A `StagingArea` is a TensorFlow data structure that stores tensors across
multiple steps, and exposes operations that can put and get tensors.
Each `StagingArea` element is a tuple of one or more tensors, where each
tuple component has a static dtype, and may have a static shape.
The capacity of a `StagingArea` is unbounded and supports multiple
concurrent producers and consumers; and provides exactly-once delivery.
Each element of a `StagingArea` is a fixed-length tuple of tensors whose
dtypes are described by `dtypes`, and whose shapes are optionally described
by the `shapes` argument.
If the `shapes` argument is specified, each component of a staging area
element must have the respective fixed shape. If it is
unspecified, different elements may have different shapes,
"""