Exclude prompt from generated response

[rasbt]

The default setting in generate/base.py returns the formatted prompt plus response, which is useful for demonstration and seeing what's going on. However, when using the model for serving, it can lead to ugly outputs like

system

You are a helpful assistant.
user

Fix typos in the following sentence: Exampel input
assistant

The corrected sentence is: "Example input".

So this PR fixes that and allows the generate function in generate/base.py to be consistent with the chat behavior.

CC @aniketmaurya