Rename arguments "--*_data_dir" to "--*_data"

[terrytangyuan]

Currently, if we want to use ODPS data source, we can pass ODPS table names to "--*_data_dir" arguments instead of previously passing file directories if we want to use RecordIO files as data source. I am proposing the following changes so the data sources are not "file"-dependent:

training_data_dir -> training_data
evaluation_data_dir -> evaluation_data
prediction_data_dir -> prediction_data

We can add help information like this:

This can be either the data directory that contains RecordIO files or an ODPS table name used for training.

If there are any suggestions on other names, please let me know in this issue as well.

[chunyang-wen]

Is it better to append a string "path" at the end?

[terrytangyuan]

@chunyang-wen But in the ODPS case, this should be a table name instead. Either a "path" or "directory" doesn't make much sense.

[chunyang-wen]

@terrytangyuan For ODPS, path=odps://project.table. Previously we use path for ODPS table. I agree with you.

[ywskycn]

Let's rename the parameter.