feat(PTv3): restore default checkpoint loading

[amadeuszsz]

Summary

In #174 I applied weight remapping, but the proper way is to do it via hook, e.g.:

hooks = [
    dict(
        type="CheckpointLoader",
        keywords="enc",
        replacement="backbone.enc",
    ),
    dict(type="IterationTimer", warmup_iter=2),
    dict(type="InformationWriter"),
    dict(type="SemSegEvaluator"),
    dict(type="CheckpointSaver", save_freq=None),
    dict(type="PreciseEvaluator", test_last=False),
]

This PR restores default implementation. Note that previous PR is not being reverted as other part of that PR was valid.