trunk_size should be chunk_size

[BoPeng]

Not sure why option trunk_size was chosen but it should be chunk_size or batch_size, I think.

[gaow]

trunk_size and trunk_workers? I thought that was one of your analogies of a collection of parallel branches of jobs from a "trunk". I've became used to these parameter names.

[BoPeng]

I guess I had "chunk" in mind but used "trunk" without further thinking. batch_size might be more appropriate anyway.

[gaow]

batch is better. But it would be best if we keep the backward compatibility of trunk

[BoPeng]

Of course we will if I ever change the parameter name.