Fix verifier kwarg leaking into draft model constructors (#669)

<!-- markdownlint-disable -->

> ⚠️ **DO NOT MERGE before #672.** This PR is stacked on #672 (remove
eagle converter) and its base is `remove-eagle-converter`. Merging this
first would pull in #672's changes prematurely and/or land against the
wrong base. Merge order: **#672 first, then this.** Once #672 merges, I
will rebase this onto `main`. The diff here is the 3-line loader change
only.

## Purpose

Fix `TypeError: Eagle3DraftModel.__init__() got an unexpected keyword
argument 'verifier'` raised during draft model construction in
`scripts/train.py --from-pretrained`.

This breaks the EAGLE3 finetuning smoke test:

```
tests/e2e/smoke/test_finetuning_sanity.py::test_finetuning_weight_sanity
TypeError: Eagle3DraftModel.__init__() got an unexpected keyword argument 'verifier'
```

`build_draft_model` passes `verifier=args.verifier_name_or_path` to
`from_pretrained` (for the external-checkpoint auto-convert path added
in #617). `SpeculatorModel.from_pretrained` had no `verifier` parameter,
so it stayed in `**kwargs`, was forwarded to HF's `from_pretrained`, and
reached the model constructor `cls(config, **kwargs)`. Draft models take
only `config` and reject it.

## Fix

Make `verifier` an explicit named parameter of
`SpeculatorModel.from_pretrained` so it is consumed at the loader
boundary (never forwarded into the constructor), used directly for
auto-conversion, and forwarded through the base-class re-dispatch
alongside `t2d`/`d2t`.

With the attachment-style `EagleSpeculator` removed in #672, no model
constructor takes `verifier` anymore, so consuming it is sufficient — no
per-model special-casing needed.

## Tests

- Failing test fixed:
`tests/e2e/smoke/test_finetuning_sanity.py::test_finetuning_weight_sanity`
(covered on CI).
- `make style` / `make quality`: pass.
- `pytest tests/unit/test_model.py
tests/unit/convert/test_entrypoints.py`: 20 passed.

## Checklist

I have filled in:

- [x] The purpose of the PR, such as "Fix some issue (link existing
issues this PR will resolve)".
- [x] The test plan/results, such as providing test command and pasting
the results.
- [ ] (Optional) The necessary documentation update.
- [x] I (a human) have written or reviewed the code in this pr to the
best of my ability.

---------

Signed-off-by: shanjiaz <hezhao@redhat.com>
Signed-off-by: Rahul-Tuli <rtuli@redhat.com>
Co-authored-by: shanjiaz <hezhao@redhat.com>

Author: rahul-tuli

src/speculators/model.py

@@ -244,6 +244,7 @@ def from_pretrained(
         weights_only: bool = True,
         t2d: torch.Tensor | None = None,
         d2t: torch.Tensor | None = None,
+        verifier: str | None = None,
         **kwargs,
     ) -> "SpeculatorModel":
         """
@@ -319,7 +320,7 @@ def from_pretrained(
 
                 pretrained_model_name_or_path = maybe_convert_external_checkpoint(
                     pretrained_model_name_or_path,
-                    verifier=kwargs.get("verifier"),
+                    verifier=verifier,
                     cache_dir=cache_dir,
                     config_dict=config_dict,
                 )
@@ -362,6 +363,7 @@ def from_pretrained(
                 weights_only=weights_only,
                 t2d=t2d,
                 d2t=d2t,
+                verifier=verifier,
                 **kwargs,
             )