Optimize CPU RAM peak memory during quantization

[lvliang-intel]

Description

Optimize CPU RAM peak memory during quantization:

1. Two optional CPU RAM optimizations, gated by [low_cpu_mem_usage]:
   cpu_stream_offload_blocks: offload block weights to disk and load them on demand during block-wise quantization, then re-offload quantized weights; restore at the end.
   cpu_stream_loss: avoid caching block outputs by computing targets on-the-fly with a frozen block copy (requires [nblocks=1]).

2. The quantization flow caches inputs once, then processes blocks sequentially, loading/offloading weights and optionally streaming loss to keep peak CPU RAM low.

Test

Quantize Qwen/Qwen3-4B-Instruct-2507 with AutoRound (4-bit) and compare CPU RAM peak usage with different optimization options.

Optimization options:

1. cpu_stream_offload_blocks: Offload block weights to disk, load on demand
2. cpu_stream_loss: Compute loss on-the-fly using frozen block copy

Summary: Peak RAM Comparison

Configuration	Peak RAM (GB)	Time (s)	RAM Saved
Baseline	24.29	1582.3	baseline
+ offload_blocks	20.26	1609.1	-4.03 GB
+ stream_loss	21.31	1364.0	-2.98 GB
All optimizations	15.57	1269.3	-8.72 GB

Type of Change

• Bug fix
• New feature
• Documentation update
• Performance improvement
• Code refactoring
• Other (please specify):

Related Issues

Fixes or relates to #

Checklist Before Submitting

• My code has been tested locally.
• Documentation has been updated as needed.
• New or updated tests are included where applicable.

[Copilot]

Pull request overview

This PR optimizes CPU RAM usage during model quantization by introducing two optional streaming strategies. The changes enable efficient quantization of large models by reducing peak memory consumption through block-wise weight offloading to disk and on-the-fly loss computation.

Changes:

• Added CPU RAM optimization options (cpu_stream_offload_blocks and cpu_stream_loss) to reduce memory usage during quantization
• Modified export logic to only save quantization config attributes that differ from scheme defaults
• Added comprehensive test for CPU RAM optimization with memory tracking

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
auto_round/compressors/base.py	Core implementation of CPU RAM optimization with block offloading and streaming loss computation
auto_round/utils/model.py	Added utility functions for saving/loading/clearing module weights to support offloading
auto_round/export/export_to_autoround/export.py	Modified to only save non-default config attributes in extra_config
auto_round/export/export_to_autoround/export_to_fp8.py	Modified to only save non-default config attributes in extra_config
auto_round/export/export_to_autoround/export_to_nvfp_mxfp.py	Modified to only save non-default config attributes in extra_config
test/test_cuda/advanced/test_cpu_ram_optimization.py	New test file to validate CPU RAM optimization features
test/test_cuda/quantization/test_mix_bits.py	Updated assertions to verify only non-default attributes are saved
test/test_cpu/quantization/test_mix_bits.py	Updated assertions to verify only non-default attributes are saved
test/test_cuda/integrations/test_sglang.py	Updated test configuration and assertions
test/test_cpu/quantization/test_act_quantization.py	Removed assertions for default config values
test/test_cuda/export/test_gguf.py	Changed device specification from integer to string format
auto_round/auto_scheme/utils.py	Added fallback device handling for string device specifications