fix(rollout): fix nested group sorting in fully async rollout

[looput]

Summary

• Support nested list[list[Sample]] groups in fully async rollout sorting.
• Recursively find the leaf Sample.index value.
• Avoid treating Python's list.index method as a sample index.

Bug Trigger Cause

The fully-async rollout path assumes that each rollout group is a flat:

list[Sample]

However, the multi-agent(examples/multi_agent) custom generation function may return multiple samples for a single input sample. After generate_and_rm_group gathers the results, the
actual structure becomes:

list[list[Sample]]

The original sorting logic directly reads .index from the first-level item:

index = getattr(sample, "index", None)

In the nested case, sample is a Python list. Since Python lists already have a built-in index() method, getattr(sample, "index") returns a
builtin_function_or_method instead of a numeric Sample.index.

The code then executes:
int(index)
which raises:

TypeError: int() argument must be a string, a bytes-like object or a real number, not 'builtin_function_or_method'

In short, the sorting logic did not support nested sample groups and mistakenly treated Python's list.index method as the sample index.

Validation

• Reproduced the original nested-group TypeError.
• Verified that the updated traversal correctly resolves the leaf sample index.
• Passed python -m py_compile slime/rollout/fully_async_rollout.py.

#2344