Callback support for the ini file

[jijoongmoon]

Currently, NNtrainer only supports the file type for the dataset in ini file. However, we also have the dataset API for the callback and It needs to be supported in ini file.

May be the format might be as below.

signature needs to be

typedef int (*my_data_gen_callback)(float **input, float **label, bool * last, void *user_data)

[train_set]
type = callback
path = my_data_gen.so
signature = my_data_gen_callback

[taos-ci]

:octocat: cibot: Thank you for posting issue #1923. The person in charge will reply soon.