arg : no n_predict = -2 for examples except for main and infill

[ngxson]

Supersede #12347 and #12323

Close #12264

I checked the code base and turns out n_predict is only support on main.cpp and infill.cpp

For server, use --no-context-shift to do the same thing, so it doesn't make sense to add n_predict == -2 support to server (which turns out to be quite messy)

[ggerganov]

We should remove -2 from the main and infill examples and use the --no-context-shift too, but we can do it later.

[Martin-Laclaustra]

Please reconsider that there is a real need for n_predict = -2 in the server example and --no-context-shift is not equivalent to stopping at the end of the context:

#12264 (comment)