use std::random_device{}() for default random seed

[dwrensha]

Currently, the default random seed is set by time(NULL), which returns an integer number of seconds. If we call llama_sampling_init() many times within a single second, then all of the sampling contexts get the same random seed. Such behavior can come as an unpleasant surprise. I encountered it recently while attempting to generate many samples via ollama, which makes use of an adapted version of llama.cpp/examples/server/server.cpp.

We can avoid the problem by using std::random_device, as is already done in utils.hpp.

Note that on old versions of libstd++ on vxworks and mingw std::random_device was sometimes deterministic, however that was considered a bug and was fixed.

I don't believe there should be any performance issues with using std::random_device{}(). On my Linux machine, it does not even require a system call, I think because it's using one of the special CPU instructions RDSEED or RDRAND.

[dwrensha]

I originally opened #6953 proposing to use std::chrono::system_clock_now(), but on further reflection I now like this PR better. Please feel free to accept either one.

[dwrensha]

Here's how I tested this PR.
In one terminal, I launched the server example:

$ ./bin/server --model ~/models/llemma_7b.Q4_0.gguf -ngl 15

and in another terminal I repeatedly made requests to the /completion endpoint:

$ for i in {1..100}; do curl -s -X POST "http://localhost:8080/completion" -H "accept: application/json" -H "Content-Type: application/json" -d '{"prompt": "The best ice cream flavor is", "stream":false, "stop": [".", "!"], "n_predict": 10}' | jq ".content"; done

With the current default random seed generator, I get lots of repeat responses, like this:

" chocolate chip"
" chocolate chip"
" chocolate chip"
" chocolate chip"
" chocolate chip"
" "
" "
" "
" "
"\n\\frac{10}{3}"
"\n\\frac{10}{3}"
" The ice cream is good The The The The"
" 4"
" 4"
" 4"
" chocolate chip if your preference is cookie flav"
" chocolate chip if your preference is cookie flav"
"\n @Vanilla Ice @Bubble g"
" the one that your customers like the most"
" the one that your customers like the most"
":\n65,8\n\n###"
" chocolate chip cookie dough ice cream"
" chocolate chip, and the best ice cre"
" chocolate chip, and the best ice cre"
" vanilla"
" vanilla"
"\n 4"
"\n 4"
" cream cheese flavor"
" cream cheese flavor"
" strawberry"
" strawberry"
" strawberry"
...

After the change in the PR, the repeats go away:

" vanilla, but the worst is buttercre"
" chocolate chip cookies and"
" vanilla ice cream"
" chocolate chip"
" peanut butter flavor ice cream"
" chocolate chip"
" chocolate chip, but you The best ice"
" 1 2 3 4 5"
" chocolate chip cookies"
" the best"
" chocolate chip, of course"
" chocolate chip"
" the best flavor for you"
"\navamegh\n\n28\n\n"
" chocolate chip cookie dough ice cream"
" chocolate chip"
" The best ice cream flavor is The best"
" vanilla"
" vanilla, but chocolate is the best"
" strawberry rancher 2 ice cream"
": The best The best flavor of ice cre"
" The The best ice cream flavor is The"
" not vanilla but strawberry ice cream"
" vanilla"
...