llama/ggml: add LLM training support

[JohannesGaessler]

See https://github.com/ggerganov/ggml/issues/1025 except I decided to implement the training directly in llama.cpp after all because the GPT-2 GGML example is already pretty complex, would require a significant amount of effort to refactor, and I'm not familiar with the codebase at all.

The goal of this PR is to add general training support to llama.cpp using ggml_opt. CPU training seems to work, other backends are missing support for some GGML ops. It's currently not possible to actually save the finetuned model to disk but you can confirm that the finetuning works by doing one epoch over the input text prior to perplexity calculation (or by observing how the loss goes down with the new finetune example). One epoch over the test set of Wikitext-2 (with the stride chosen in such a way that each token is used twice per epoch) currently takes ~1 minute with Stories 260k or ~20 hours and ~100 GB RAM with LLaMA 3 8b. For the user-facing API my concrete plans are:

• The parameter n_ctx determines the max. sequence length with which the model is trained.
• The parameter n_batch determines how many tokens are consumed per optimizer step.
• The parameter n_ubatch determines the number of tokens in parallel, enables speed <-> memory use tradeoff, should have no effect on the result except for differences in floating point rounding error.
• A function with which the user can initialize a dataset from type std::vector<llama_token>. Currently I have this as part of llama.h but maybe this would make more sense to put in common.h?
• A function llama_opt_init that prepares a llama_context for training and lets the user define things like the learning rate or which tensors should be trainable parameters.
• A function llama_opt_epoch that performs one epoch over a ggml_opt_dataset, equivalent to ggml_opt_epoch.
• Maybe a function like llama_opt_fit equivalent to ggml_opt_fit that is even more high-level?

Currently, while functional, the PR is in a bad state in terms of software design and is in need of a refactor. The reason I'm already opening it now is because I want to ask for advice regarding how to best implement llama_opt_epoch. My current approach was to try and hijack the first half of llama_decode_internal but I found that in the end all I needed from it was the generation of the next llama_ubatch and the corresponding manipulation of the KV cache. But maybe it would make more sense to instead write a function like llama_prepare_next_ubatch and to use that function in llama_decode_internal and llama_opt_epoch?