selective_mixed_precision: QKV-aware overrides, AUTO memory mode, MULTI_GPU dispatch (microsoft#2475)

## Describe your changes
Based on microsoft#2473

## Checklist before requesting a review
- [ ] Add unit tests for this change.
- [ ] Make sure all tests can pass.
- [ ] Update documents if necessary.
- [ ] Lint and apply fixes to your code by running `lintrunner -a`
- [ ] Is this a user-facing change? If yes, give a description of this
change to be included in the release notes.

## (Optional) Issue link

---------

Co-authored-by: Sunghoon Choi <sunghcho@microsoft.com>
Co-authored-by: Sunghoon Choi <35605090+hanbitmyths@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

olive/passes/pytorch/quant_utils.py

@@ -5,6 +5,7 @@
 from __future__ import annotations
 
 import logging
+from copy import deepcopy
 from dataclasses import dataclass
 from typing import TYPE_CHECKING, Callable
 
@@ -14,6 +15,7 @@
 from olive.common.quant.hf_utils import (
     OliveHfQuantizationConfig,
     OliveHfQuantizationMethod,
+    OliveHfQuantizationOverrideConfig,
     replace_matching_submodules,
     tie_quant_word_embeddings,
 )
@@ -78,6 +80,106 @@ def get_quantizer_config(allow_embeds: bool = False) -> dict[str, PassConfigPara
     }
 
 
+def get_qkv_quantization_groups(wrapper: ModelWrapper, module_names: set[str] | None = None) -> list[tuple[str, ...]]:
+    """Get attention input projection groups that must share quantization settings.
+
+    Names are resolved from ``wrapper.model.named_modules()`` to stay correct for any layer
+    container (``ModuleList``, ``ModuleDict``, custom containers) and for unpacked QKV
+    submodules. When ``module_names`` is provided, attention inputs not in the set are
+    dropped from the group. Groups with fewer than two members are skipped.
+    """
+    module_to_name = {id(module): name for name, module in wrapper.model.named_modules()}
+    qkv_groups = []
+    for layer_wrapper in wrapper.get_layer_wrappers():
+        attn_inputs, _ = layer_wrapper.get_attention_inputs()
+        group = tuple(
+            name
+            for name in (module_to_name.get(id(module)) for module in attn_inputs)
+            if name is not None and (module_names is None or name in module_names)
+        )
+        if len(group) > 1:
+            qkv_groups.append(group)
+    return qkv_groups
+
+
+def _quant_config_rank(qargs: dict[str, int | bool]) -> tuple[int, int, int]:
+    """Rank quantization configs by precision; higher rank means more precise.
+
+    Ordering: higher ``bits`` wins; among equal bits, smaller positive ``group_size`` wins;
+    per-channel (``-1``) wins over per-tensor (``0``) but loses to positive group sizes.
+    ``symmetric`` is intentionally not part of the ordering since it is a representation
+    choice rather than a strict precision axis.
+    """
+    bits = qargs["bits"].value if hasattr(qargs["bits"], "value") else qargs["bits"]
+    group_size = qargs["group_size"]
+    if group_size > 0:
+        group_size_rank = (2, -group_size)
+    elif group_size == -1:
+        group_size_rank = (1, 0)
+    else:
+        group_size_rank = (0, 0)
+    return bits, *group_size_rank
+
+
+def normalize_qkv_quant_config(
+    wrapper: ModelWrapper,
+    qcfg: OliveHfQuantizationConfig,
+    locked_modules: set[str] | None = None,
+) -> OliveHfQuantizationConfig:
+    """Promote split QKV projection overrides to one shared quantization config.
+
+    Groups span all attention input projections of a layer regardless of whether the current
+    pass quantizes them; follow-up passes (e.g. RTN after AutoClip) will pick up the shared
+    settings via the recorded overrides so downstream QKV fusion remains valid.
+
+    ``locked_modules`` are modules whose overrides must not be rewritten -- typically the
+    pre-existing overrides of an already-quantized checkpoint. For a group containing a
+    locked member, the shared config is forced to that locked member's config; if multiple
+    locked members of one group disagree, the group is left untouched.
+    """
+    locked_modules = locked_modules or set()
+    for group in get_qkv_quantization_groups(wrapper):
+        group_qargs = {name: qcfg.get_qlinear_init_args(name) for name in group}
+        if len({tuple(qargs.items()) for qargs in group_qargs.values()}) == 1:
+            continue
+
+        locked_in_group = [name for name in group if name in locked_modules]
+        locked_configs = {tuple(group_qargs[name].items()) for name in locked_in_group}
+        if len(locked_configs) > 1:
+            logger.debug(
+                "QKV group %s contains already-quantized members with conflicting configs; "
+                "skipping (downstream QKV fusion may be inhibited).",
+                group,
+            )
+            continue
+        promoted_qargs = (
+            group_qargs[locked_in_group[0]] if locked_in_group else max(group_qargs.values(), key=_quant_config_rank)
+        )
+
+        logger.debug("Promoting QKV group %s to shared quantization config %s", group, promoted_qargs)
+        for name in group:
+            if name in locked_modules:
+                continue
+            override = {k: v for k, v in promoted_qargs.items() if getattr(qcfg, k) != v}
+            if override:
+                qcfg.overrides[name] = OliveHfQuantizationOverrideConfig(**override)
+            else:
+                qcfg.overrides.pop(name, None)
+
+    return qcfg
+
+
+def _collect_excluded_attn_inputs(wrapper: ModelWrapper) -> set[torch.nn.Module]:
+    excluded: set[torch.nn.Module] = set()
+    for layer_wrapper in wrapper.get_layer_wrappers():
+        attn_inputs, _ = layer_wrapper.get_attention_inputs()
+        if len(attn_inputs) == 1:
+            excluded.add(attn_inputs[0])
+        else:
+            excluded.update(attn_inputs[:2])
+    return excluded
+
+
 def prepare_model(
     model: HfModelHandler,
     config: type[BasePassConfig],
@@ -99,42 +201,65 @@ def prepare_model(
     if existing_qcfg := getattr(model.get_hf_model_config(), "quantization_config", None):
         if not allow_quantized:
             raise ValueError("Model is already quantized. Cannot quantize again using this pass.")
-        if not isinstance(existing_qcfg, dict):
-            existing_qcfg = existing_qcfg.to_dict()
+        # Always work on a fresh copy: the underlying HF config holds the original object
+        # (dict or dataclass) and we mutate ``existing_qcfg`` heavily below.
+        existing_qcfg = deepcopy(existing_qcfg) if isinstance(existing_qcfg, dict) else existing_qcfg.to_dict()
         if existing_qcfg.get("quant_method", None) != OliveHfQuantizationMethod.OLIVE:
             raise ValueError("Model has an existing quantization configuration that is not compatible with this pass.")
 
     wrapper = ModelWrapper.from_model(load_hf_base_model(model))
     wrapper.model.eval()
 
-    qcfg = get_quant_config(model, config)
+    excluded_attn_inputs = _collect_excluded_attn_inputs(wrapper) if exclude_attn_inputs else set()
+
+    fresh_qcfg = normalize_qkv_quant_config(wrapper, get_quant_config(model, config))
 
     originally_tied_embeddings = wrapper.config.tie_word_embeddings
-    if qcfg.lm_head or qcfg.embeds:
+    if fresh_qcfg.lm_head or fresh_qcfg.embeds:
         wrapper.maybe_untie_word_embeddings()
 
     lm_head_name = wrapper.get_lm_head()[1]
     embeds_name = wrapper.get_embeds()[1][0]
-    new_qargs: dict[str, dict[str, int | bool]] = {}
-
-    excluded_attn_inputs: set[torch.nn.Module] = set()
-    if exclude_attn_inputs:
-        for layer_wrapper in wrapper.get_layer_wrappers():
-            attn_inputs, _ = layer_wrapper.get_attention_inputs()
-            if len(attn_inputs) == 1:
-                excluded_attn_inputs.add(attn_inputs[0])
-            else:
-                excluded_attn_inputs.update(attn_inputs[:2])
 
     def should_quantize(module: torch.nn.Module, name: str) -> bool:
         if module in excluded_attn_inputs:
             return False
         if isinstance(module, torch.nn.Linear):
-            return name != lm_head_name or qcfg.lm_head
-        if qcfg.embeds and isinstance(module, torch.nn.Embedding):
+            return name != lm_head_name or fresh_qcfg.lm_head
+        if fresh_qcfg.embeds and isinstance(module, torch.nn.Embedding):
             return name == embeds_name
         return False
 
+    # Pre-existing quantized weights are immutable. If we're merging with an existing
+    # checkpoint, build the final qcfg first (merge fresh into existing, then renormalize
+    # QKV with already-quantized modules locked) so that the quant_info we attach below
+    # uses the same settings the on-disk fusion will require. Every module that is already
+    # a QuantLinear/QuantEmbedding after load is on-disk-immutable, including those that
+    # used the existing config's defaults (no explicit override entry).
+    on_disk_overrides: set[str] = set()
+    already_quantized: set[str] = set()
+    if existing_qcfg:
+        on_disk_overrides = set((existing_qcfg.get("overrides") or {}).keys())
+        already_quantized = {
+            name for name, module in wrapper.model.named_modules() if isinstance(module, (QuantLinear, QuantEmbedding))
+        }
+        fresh_names = {name for name, module in wrapper.model.named_modules() if should_quantize(module, name)}
+        merged = existing_qcfg
+        merged["overrides"] = existing_qcfg.get("overrides") or {}
+        for name in fresh_names:
+            qargs = fresh_qcfg.get_qlinear_init_args(name)
+            override = {k: v for k, v in qargs.items() if merged[k] != v}
+            if override:
+                merged["overrides"][name] = override
+        merged["lm_head"] |= fresh_qcfg.lm_head
+        merged["embeds"] |= fresh_qcfg.embeds
+        qcfg = OliveHfQuantizationConfig(**merged)
+        qcfg = normalize_qkv_quant_config(wrapper, qcfg, locked_modules=already_quantized)
+    else:
+        qcfg = fresh_qcfg
+
+    new_qargs: dict[str, dict[str, int | bool]] = {}
+
     def add_quant_info(module: torch.nn.Module, name: str) -> torch.nn.Module:
         # TODO(jambayk): validate that the module and config are compatible
         qargs = qcfg.get_qlinear_init_args(name)
@@ -144,23 +269,15 @@ def add_quant_info(module: torch.nn.Module, name: str) -> torch.nn.Module:
 
     replace_matching_submodules(wrapper.model, should_quantize, add_quant_info, description="Preparing model")
 
-    # remove overrides for modules not being quantized
+    # Drop overrides for modules that won't be quantized this pass. Pre-existing (on-disk)
+    # overrides are preserved verbatim since they describe already-quantized weights.
+    # QKV-group overrides for modules excluded from this pass are not kept: when the
+    # follow-up pass runs, the quantized members in the group will be locked and pull the
+    # remaining members back into the shared config via ``normalize_qkv_quant_config``.
     for name in list(qcfg.overrides or {}):
-        if name not in new_qargs:
+        if name not in new_qargs and name not in on_disk_overrides:
             qcfg.overrides.pop(name)
 
-    # merge the new_quant_settings into the existing quant_config
-    if existing_qcfg:
-        merged_qcfg_dict = existing_qcfg
-        merged_qcfg_dict["overrides"] = existing_qcfg.get("overrides") or {}
-        for name, qargs in new_qargs.items():
-            override = {k: v for k, v in qargs.items() if merged_qcfg_dict[k] != v}
-            if override:
-                merged_qcfg_dict["overrides"][name] = override
-        merged_qcfg_dict["lm_head"] |= qcfg.lm_head
-        merged_qcfg_dict["embeds"] |= qcfg.embeds
-        qcfg = OliveHfQuantizationConfig(**merged_qcfg_dict)
-
     word_embeddings_eligible_for_tieing = (
         originally_tied_embeddings
         and embeds_name in new_qargs