Environment

• Hardware Environment(Ascend/GPU/CPU):

Uncomment only one /device <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/device cpu

• Software Environment:
  -- MindSpore version (source or binary): master 2021/9/23 commit_id = ''[sha1]:5213c04a,[branch]:(HEAD->master,origin/master,origin/HEAD)''
  -- Python version (e.g., Python 3.7.5): Python 3.7.5
  -- OS platform and distribution (e.g., Linux Ubuntu 16.04): Ubuntu 18.04
  -- GCC/Compiler version (if compiled from source):

Related testcase

test_ms_usability_api_0001

Steps to reproduce the issue

1. check https://mindspore.cn/docs/api/zh-CN/master/api_python/mindspore/mindspore.Model.html#mindspore.Model
2. check https://mindspore.cn/docs/api/zh-CN/master/api_python/mindspore/mindspore.DatasetHelper.html#mindspore.DatasetHelper

Describe the current behavior

1. Model中eval 和train 里的Note有语法问题
2. DatasetHelper有语法问题以及示例代码可以再优化

Describe the expected behavior

1. 修复语法问题
2. DatasetHelper示例代码再详细点, 抽象一个Net，写出数据传入Net的代码

Related log / screenshot

Special notes for this issue

Model和DatasetHelper接口文档需要优化