[hotfix] fix for loading after_scheduler state

[imgaojun]

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related issue #5119

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I have introduced a new method, load_state_dict, in the Scheduler class in colossalai/nn/lr_scheduler/delayed.py. This method enhances the scheduler's ability to reinstate its state, specifically focusing on the after_scheduler component. The key changes include:

Updating the Scheduler class with the load_state_dict method.
Ensuring the after_scheduler's state is appropriately loaded from the provided state dictionary if the after_scheduler_dict key is present.

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The code coverage for the changed files is 39%.

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Name                                    Stmts   Miss  Cover
-----------------------------------------------------------
colossalai/nn/lr_scheduler/delayed.py     125     76    39%
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TOTAL                                     125     76    39%