[BUG] ConstantGradScaler and loss-scale argument not match

[BeingGod]

Describe the bug
The usage and description of loss-scale is inconsistent. The argument of loss-scale expect to get a number of positive power of 2 but ConstantGradScaler set loss-scale to real scale directly rather than 2**loss-scale.

Argument Description:

Argument Usage:

To Reproduce
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Expected behavior
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Environment (please complete the following information):

• Megatron-LM commit ID
• PyTorch version
• CUDA version
• NCCL version

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