Add a progress bar for prompt evaluation

[oobabooga]

This PR adds a tqdm progress bar that informs the total number of steps, the number of steps so far, and the remaining time during prompt evaluation.

• The progress bar is initialized with leave=False, so it disappears after completing without leaving an extra blank line in the terminal.
• It only appears when len(tokens) > 1, to keep it contained to the prompt evaluation stage.

I have been using this as a monkey patch in my project for a while now and it works great. Since it seemed as something of general interest, I have decided to submit a PR for your consideration.

[randoentity]

+1 I love this feature. It makes it so much more clear what to expect.
To hijack your hijack, I was wondering if you could help me figure out how the prompt evaluation "works" in whatever level of detail because I'm on a journey to understand it a bit more and it led me here.

In particular I'm interested how to use this with "plain" llama.cpp.

I know it's all here in the code, so I'll be searching and reading some more, and I'm lazy but receiving other people's perspective makes the experience so much richer, for the community 😉

Edit: okay I think I get it now: #893 (comment)
And after years I get to https://github.com/ggerganov/llama.cpp/tree/master/examples/main#prompt-caching
My humble appreciation to everyone that has abstracted these (and many other things that are way over my head) away and made this technology accessible.