[moe] add checkpoint for moe models

[1SAA]

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[github-actions]

The code coverage for the changed files is 73%.

Click me to view the complete report

Name                                    Stmts   Miss  Cover
-----------------------------------------------------------
colossalai/nn/layer/moe/__init__.py         6      0   100%
colossalai/nn/layer/moe/checkpoint.py      30      0   100%
colossalai/nn/layer/moe/experts.py        127     64    50%
colossalai/nn/layer/moe/layers.py         103     18    83%
tests/test_moe/test_moe_checkpoint.py      41      1    98%
-----------------------------------------------------------
TOTAL                                     307     83    73%