docs: add model spec (unstable) version (#457)

* docs: add model spec (unstable) version

* add link to hf tokenizers

MODEL_SPEC.md

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+# Tabby Model Specification (Unstable)
+
+Tabby organizes the model within a directory. This document provides an explanation of the necessary contents for supporting model serving. An example model directory can be found at https://huggingface.co/TabbyML/StarCoder-1B
+
+The minimal Tabby model directory should include the following contents:
+
+```
+ctranslate2/
+ggml/
+tabby.json
+tokenizer.json
+```
+
+### tabby.json
+
+This file provides meta information about the model. An example file appears as follows:
+
+```js
+{
+    "auto_model": "AutoModelForCausalLM",
+    "prompt_template": "<PRE>{prefix}<SUF>{suffix}<MID>"
+}
+```
+
+The **auto_model** field can have one of the following values:
+- `AutoModelForCausalLM`: This represents a decoder-only style language model, such as GPT or Llama.
+- `AutoModelForSeq2SeqLM`: This represents an encoder-decoder style language model, like T5.
+
+The **prompt_template** field is optional. When present, it is assumed that the model supports [FIM inference](https://arxiv.org/abs/2207.14255).
+
+One example for the **prompt_template** is `<PRE>{prefix}<SUF>{suffix}<MID>`. In this format, `{prefix}` and `{suffix}` will be replaced with their corresponding values, and the entire prompt will be fed into the LLM.
+
+### tokenizer.json
+This is the standard fast tokenizer file created using [Hugging Face Tokenizers](https://github.com/huggingface/tokenizers). Most Hugging Face models already come with it in repository.
+
+### ctranslate2/
+This directory contains binary files used by the [ctranslate2](https://github.com/OpenNMT/CTranslate2) inference engine. Tabby utilizes ctranslate2 for inference on both `cpu` and `cuda` devices.
+
+With the [python package](https://pypi.org/project/ctranslate2) installed, you can acquire this directory by executing the following command in the HF model directory:
+
+```bash
+ct2-transformers-converter --model ./ --output_dir ctranslate2 --quantization=float16
+```
+
+*Note that the model itself must be compatible with ctranslate2.*
+
+### ggml/
+This directory contains binary files used by the [llama.cpp](https://github.com/ggerganov/llama.cpp) inference engine. Tabby utilizes ctranslate2 for inference on the `metal` device.
+
+Currently, only `q8_0.gguf` in this directory is in use. You can refer to the instructions in llama.cpp to learn how to acquire it.