Add GenAI packages

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**Describe the solution you'd like**
The `GenAI` packages will provide torchsharp implementation for a series of popular GenAI models. The goal is to load the same weight from the corresponding python regular model.

- [x] Add design doc (#7170)
- [x] Add `Microsoft.ML.GenAI.Core` (#7177)

The following models will be added in the first wave
- [x] Phi-3 (`Microsoft.ML.GenAI.Phi`) #7184
- [x] Add README to `Microsoft.ML.GenAI.Phi` project #7206
- [x] LLaMA (`Microsoft.ML.GenAI.LLaMA`) #7220
- [ ] Mistral (`Microsoft.ML.GenAI.Mistral`)
- [x] Mistral-7b-instruct v3
- [ ] Mistral-nemo
- [x] Generate Embedding from CausalLMModel #7227
- [ ] Stable Diffusion (`Microsoft.ML.GenAI.StableDiffusion`)

MEAI intergration
- [x] Add CausalLMPipelineChatClient #7270

Fine-tuning #7287

Along with the benchmark
- [ ] Benchmark for Phi-3

- [ ] Flash Attention support #7238

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