Update type hints and doc strings for moe_utils.py

[JavaZeroo]

Added type hints for logits and z_loss_coeff in z_loss_func.

@yaox12 Update type hints and doc strings for moe_utils.py

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[yaox12]

/ok to test b1d74ae

[yaox12]

/ok to test 2747a95

[yaox12]

/ok to test 9e66cd3

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