fix swift package + add example swift implementation of batched api

[zshannon]

hey there, thanks for an amazing repo!

the swift package stopped compiling for me and it was tough to debug because the automated tests here are passing, so thought it made sense to add an example use in swift to surface the compiler error and make it trackable as part of the repo. plus, a few people have asked for swift examples with the batch api.

1. the compiler bug is fixed by including ggml-backed.c in the SPM sources, so that's the change in Package.swift. you can verify it's needed if you like by commenting out the line and running make in examples/swift.

2. i wrote a rough line for line copy of examples/batched.cpp in swift, though i don't know c++ very well so relied heavily on copilot.. seems to produce the same output but ymmv :)

[jhen0409]

I think we need figure out why CI is still passing after #3548 merged, it seems like the xcodebuild will ignore sources and build the llama scheme directly, and the sources are used when llama is a dependency. If we can't figured it out, I think adding the swift example build to CI is also an option. Great work btw!

UPDATE: Since it includes ggml-backend.h it will build successfully in CI, so that should be expected behavior. However, undefined symbol error will occur when referencing externally if not add ggml-backend.c.

[zshannon]

Derp something is wrong with my swift string parsing, doesn't work for emojis

[ggerganov]

Thanks for the contribution! Is this example going to be built by the CI?
If not, we should look into adding build support, otherwise it will get obsoleted with API changes in the future

[zshannon]

rebased master + added the build step to the root makefile when running on macOS so the CI will pick it up.

[zshannon]

Ok, I'll add a patched GitHub action with those changes

[jhen0409]

Looks good! 👍