fit: warn when --cpu-moe is used on integrated/UMA GPUs; docs: add Jetson MTP benchmark

[hernandez42]

Summary

Two changes:

1. common/fit.cpp — iGPU warning for --cpu-moe

When --cpu-moe would move MoE experts out of GPU memory, add a runtime warning if the device is an integrated GPU (GGML_BACKEND_DEVICE_TYPE_IGPU). On iGPU/UMA systems such as the NVIDIA Jetson AGX Xavier, moving MoE weights to system memory forces their computation onto the CPU, which is ~20x slower than GPU compute (~6 t/s vs ~23 t/s). The warning suggests omitting --cpu-moe on integrated GPUs.

Background: CUDA's cuDeviceGetAttribute for CU_DEVICE_ATTRIBUTE_INTEGRATED is known to return incorrect values in some driver versions (see #15034), so this change works at the ggml_backend_dev_type level instead — GGML_BACKEND_DEVICE_TYPE_IGPU is set correctly by ggml's CUDA backend enumeration.

2. docs/speculative.md — Jetson AGX Xavier MTP benchmark data

Adds a new Multi Token Prediction (draft-mtp) section with real-world benchmark data from an NVIDIA Jetson AGX Xavier (Volta 512-core, ~135 GB/s unified memory, 32 GB shared RAM, 30W TDP).

Key findings documented:

• MTP with --speculative-max 2 achieves 23.14 t/s (+48%) with 97.7% draft acceptance rate
• The default --speculative-max 3 produces ~75% acceptance on embedded GPUs — lowering to 2 is optimal
• --cpu-moe on iGPU/UMA systems causes ~4x slowdown vs GPU-only (6 vs 23 t/s)

Motivation

NVIDIA Jetson devices (AGX Xavier, AGX Orin, NX, Nano) are increasingly popular for local LLM inference. The embedded GPU community lacks documented MTP performance data and can easily hit the --cpu-moe performance trap. These changes help users on integrated GPUs avoid common pitfalls.

[ggml-gh-bot]

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[hernandez42]

Thanks for the automated check!

1. PR Template: I went back and reviewed the template — I see the required sections were missing. I have since updated the PR description to follow the template.

2. AI-assisted: To clarify, the fit.cpp change is entirely original work — it translates the GGML_BACKEND_DEVICE_TYPE_IGPU enum into a simple runtime warning in the --cpu-moe decision path. The benchmark data in speculative.md was collected on a physical NVIDIA Jetson AGX Xavier (32 GB unified memory, Volta 512-core @ ~135 GB/s, 30W TDP) using the exact parameters documented. If you need any additional reproducibility info, let me know!

Happy to make any further changes needed to get this merged.