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.
Change from system Python to environment python3. For Ubuntu 20.04, only `python3` is
available and there's no `python`. So at least `python3` is consistent with Python
virtual env, Ubuntu and anaconda.
CLI is not most developed interface. Putting them into correct directory can help new users to avoid it as most of the use cases are from a language binding.
