Refactor FashionMNIST notebook to use next() for data iterator

[k22036]

Fixes #{issue number}

Description:

• fixed iterator usage
• fixed plotting code
  • Changed division operator '/' to integer division '//' in the fig.add_subplot method

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[vfdev-5]

Thanks @k22036 ! The changes make sense.