Previously, we use `libsvm` as default when format is not specified. However, the dmlc
data parser is not particularly robust against errors, and the most common type of error
is undefined format.
Along with which, we will recommend users to use other data loader instead. We will
continue the maintenance of the parsers as it's currently used for many internal tests
including federated learning.
* Some minor changes to the code style
Some minor changes to the code style in file basic_walkthrough.py
* coding style changes
* coding style changes arrcording PEP8
* Update basic_walkthrough.py
