[Refactor] Move fix_id_list to Retriever

[gaotongxiao]

Motivation

The prior design that decoupled fix_id_list from FixKRetriever was terrible and resulted in code redundancy in Inferencers. It also hampered the development of some incoming features.

BC-breaking

This PR breaks some custom config where FixKRetriever is used, but the effect may not be significant, and the migration cost is acceptable. Specifically, users would need to change their configs from

xxx_infer_cfg = dict(
    ...
    retriever=dict(type=FixKRetriever),
    inferencer=dict(type=PPLInferencer, fix_id_list=[0, 1, 2, 3, 4]))

to

xxx_infer_cfg = dict(
    ...
    retriever=dict(type=FixKRetriever, fix_id_list=[0, 1, 2, 3, 4])),
    inferencer=dict(type=PPLInferencer)

Which is quite straightforward.