fix(onnx): make provider_options optional in OnnxRuntimeModel._from_pretrained (#13653)

[Anai-Guo]

Problem

Fixes Issue 1 of #13653.

OnnxRuntimeModel._from_pretrained() calls kwargs.pop("provider_options") without a default in both the local-directory and Hub branches. Pipeline loading happens to pass provider_options=None, so the failure is hidden in that path. Direct use of the public helper, however, raises KeyError before any loading happens.

Reproduction

import tempfile
from diffusers.pipelines.onnx_utils import OnnxRuntimeModel

with tempfile.TemporaryDirectory() as d:
    OnnxRuntimeModel.from_pretrained(d)
# KeyError: 'provider_options'

Fix

Pass a None default to both kwargs.pop("provider_options", None) calls. The downstream OnnxRuntimeModel.load_model() already normalises None → [] (lines 86-89), so the new default keeps existing pipeline-loading behaviour identical and unblocks direct from_pretrained() calls.

Notes

• Two-line change, scoped strictly to lines 185 and 205 of onnx_utils.py.
• No new tests added — the maintainer's review note already flags missing test coverage for direct OnnxRuntimeModel.from_pretrained() and lists this in the meta-issue test-coverage section, which felt like an explicit follow-up rather than something to slip into this surgical fix. Happy to add a fast unit test if preferred.

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