Design for serving models with different backends

This commit introduces a new design document for the [`ilab model
serve`] command, detailing its
functionality to serve models using different backends, specifically
`llama-cpp` and `vllm`. It outlines the command structure, including
backend-specific flags and arguments, and proposes a testing strategy
for new backend integrations.

Co-authored-by: Sébastien Han <[email protected]>
Signed-off-by: Ali Maredia <[email protected]>

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docs/cli/ilab-model-serve-backend.md

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+# Design for `ilab model serve` command with backend support
+
+## Background
+
+With the [request from the community](https://github.com/instructlab/instructlab/issues/1106) for `ilab` to serve different backends such as [vllm](https://docs.vllm.ai/en/stable/) and the [cli redesign](ilab-model-engine.md), this design doc's purpose is to flesh out the behavior of the `ilab model serve` command.
+
+Specifically, this doc addresses the design of subcommands of `ilab model serve` that apply to
+different serving backends.
+
+## Design
+
+### Backend
+
+Since the subject of the `ilab model serve` command is a model, regardless of the format of the model, every command takes in the `--model` flag or uses its default value in the config.
+
+`ilab model serve` has a new flag `--backend` that will be used to serve models with. As of this design, the two backends `ilab` would serve with are `llama-cpp` and `vllm`.
+
+This would lead to the commands:
+
+- `ilab model serve --backend llama-cpp`
+- `ilab model serve --backend vllm`
+
+There are specific flags for `ilab model serve` that would apply to all backends. These can be viewed by running `ilab model serve --help`.
+For the specific subcommands, each has their own flags that can be viewed via `ilab model serve <BACKEND> --help`.
+This ensures there is no mix up of flags between backends.
+
+The following is an overview for the flags of `ilab model serve`:
+
+```console
+ilab model serve
+|
+|_______ (backend agnostic flags)
+|
+|_______ --backend ['llama-cpp', 'vllm']
+|_______ --backend-args
+```
+
+The `backend` flag will also be available as an option in the config file (`config.yaml`). This will allow users to
+set a default backend for `ilab model serve` in the config. Also, commands like `ilab model chat`
+and `ilab data generate` that serve models in the background will use the default backend specified
+in the config. Here is an example of what the config file would look like:
+
+```yaml
+serve:
+  gpu_layers: -1
+  host_port: 127.0.0.1:8000
+  max_ctx_size: 4096
+  model_path: models/merlinite-7b-lab-Q4_K_M.gguf
+  backend: llama-cpp
+```
+
+### Backend flags
+
+The `--backend-args` flag is a string that will be passed to the backend as arguments. This flag is
+used to pass backend-specific arguments to the backend.
+
+It will also be available as an option in the config file (`config.yaml`). This will allow users to set default backend arguments for `ilab model serve` in the config. Here is an example of what the config file would look like:
+
+```yaml
+serve:
+  backend: llama-cpp
+  backend_args:
+    num_gpu_layers: 4
+    max_ctx_size: 1024
+```
+
+For clarity and ease of implementation, when using the `--backend-args` flag, the user must pass the
+`--backend` flag as well. This is to ensure that the backend-specific arguments are passed to the
+correct backend. Any backend-specific arguments that are not passed to the correct backend will be
+reported as an error.
+
+## Command Examples
+
+### Bare-bones but model specific command
+
+```shell
+ilab model serve --model <PATH>
+```
+
+- Serves the model at `<PATH>`.
+- If the `<PATH>` is the path for a model that can be run by `llama-cpp` then `llama-cpp` is automatically used as the model serving backend.
+- If the `<PATH>` is the path for a model that can be run by `vllm` then `vllm` is automatically used as the model serving backend.
+- If the model at `<PATH>` can be run by either backend, then a default backend defined in the
+  config will be used as the model serving backend. In the case where there is ambiguity and a setting is not defined, a hardcoded preference will be used (all currently supported providers do not have this issue). A future profile specification will likely replace the hardcoded fallback.
+
+### Bare-bones command
+
+```shell
+ilab model serve
+```
+
+- This command has the same behavior as the one above but the `--model` is whatever the default model path is in the config. This is the existing behavior of `ilab serve` today.
+
+### Llama-cpp backend specific commands
+
+```shell
+ilab model serve --model <PATH> --backend llama-cpp --backend-args '--num-gpu-layers 4, --max-ctx-size 1024'
+```
+
+- This command serves a model with `llama-cpp`.
+- If the model provided is not able to be served by llama-cpp, this command would error out and suggest an alternate backend to use.
+- The existing flags to `ilab serve` (besides `--model-path` & `--log-file`) are now specific to the llama-cpp backend.
+
+### vllm backend specific commands
+
+```shell
+ilab model serve --model <PATH> --backend vllm --backend-args '--chat-template <PATH>, <MORE VLLM SPECIFIC ARGS>'
+```
+
+- This command serves a model with `vllm`.
+- If the path provided is not able to be served by `vllm`, this command would error out and suggest an alternate backend to use.
+- There are [dozens](https://docs.vllm.ai/en/latest/serving/openai_compatible_server.html#command-line-arguments-for-the-server) of flags for vllm. Whichever arguments the community deems the most important to include, will be added as flags to `ilab model serve vllm`.
+- Any remaining arguments can be specified in the value of the flag `--vllm-args`.
+
+## Testing
+
+An additional end-to-end test will be added that for a new backend for `ilab model serve`. This new test should be triggered whenever code changes to the new backend serving code are made or before a release.
+
+This new test will do the following:
+
+1. Initialize ilab in a virtual env via `ilab config init`.
+2. Download a model via `ilab model download`.
+3. Serve the downloaded model with the new backend via `ilab model serve`.
+4. Generate synthetic data using the served model via `ilab data generate`.
+5. Chat with the served model via `ilab model chat`.
+6. Any future commands that interact with a served model should be added to the test.
+
+Some commands, like `ilab model chat` and `ilab data generate`, serve models in the background as part of the command. If automatic serving of a new backend is implemented for a command, testing of that command that will also be included in the new end-to-end test.
+
+## Handling existing backend-specific commands
+
+The existing `ilab model serve` command has flags that are specific to the `llama-cpp` backend. The current list of flags are:
+
+- `--num-gpu-layers`
+- `--max-ctx-size`
+- `--num-threads`
+
+These flags will be moved to `--backend-args` and will be used as the default arguments for
+`llama-cpp` backend. This will allow for a more consistent experience across backends. The flag will
+be supported up to two releases after the release of the new backend. After that, the flag will be
+removed. During the two releases, a warning will be printed to the user when the flag is used.