fix #1224 reverse prompt and multi line

[newTomas]

Fixes #1224
In the original code, when an empty line is encountered, it stops the user from entering, although in reality, an empty line can be a side effect of the previous input and should simply be ignored.

[DeveloperOl]

works great for me :)

[newTomas]

Tested with a wider set of models and found 1 more bug in another part of the code. If the antiprompt is "User: ", and the model generated the tokens "User", ":", " How", then the program takes the last n characters, where n is the length of the antiprompt, and it turns out that it compares "User: " and "r: How" and does the conclusion that the antiprompt was not met.
I'll try to make a fix tomorrow. Or someone else can do it, it's a bit more difficult than the previous bug.
https://github.com/ggerganov/llama.cpp/blob/master/examples/main/main.cpp#L514

[ggerganov]

@newTomas

The fix for this issue is not very trivial.
One has to check for the described situation and decrease n_past by 1. And also somehow erase the extra generated token (i.e. " How" in your example).

To resolve this, we introduced the --input-prefix argument that can be used to inject the extra space.
Let's merge this PR like it is after the indentation fixes

[newTomas]

@ggerganov
I approved your commit. Let's merge

[agronholm]

Unfortunately, I discovered that not only did this not fix the problem, it made it a whole lot worse. Previously, I was able to at least get it to produce more tokens by pressing enter. Now even that won't work.