fix: handle dict output in SmolInstructDataset.load for SIDER subset

[octo-patch]

Fixes #2440

Problem

The property_prediction-sider subset of SMolInstruct stores the output field as a multi-label dict (one entry per side-effect category) rather than a plain string like every other subset. When HuggingFace Arrow tries to build a Dataset from rows that mix string and struct-typed outputs, it raises:

TypeError: Couldn't cast array of type struct<Hepatobiliary disorders: string, ...> to string

Additionally, random was used by the existing mini_set sampling code but was never imported, which would cause a NameError whenever mini_set=True.

Solution

In opencompass/datasets/smolinstruct.py:

1. Before appending each row to raw_data, check whether the output field is a dict; if so, serialize it to a JSON string. This ensures Dataset.from_list() always receives a uniform string-typed column regardless of the subset.
2. Add the missing import json and import random statements.

Testing

The fix can be verified by running the pp-acc evaluation config that includes the SIDER subset:

opencompass config.py --reuse

where config.py imports mini_pp_acc_datasets_0shot_instruct (which includes PP-SIDER-0shot-instruct-mini). Previously this raised a TypeError; with this fix the dataset loads cleanly.