Possible bugs in the data loading functions

[PeterZhizhin]

First, we read B*T+1 tokens, but advance the iterator by B*T tokens instead.

Then, there is this if statement:

    if (loader->current_position + (loader->num_processes * B * T + 1) * sizeof(int) > loader->file_size)

Possibly, we should remove the loader->num_processes multiplication here.

We need to verify that this is the way these functions should work.