Safe accumulation type for nd.norm

[eric-haibin-lin]

For FP16, reduction operators tend to loss precision if the accumulation data type remains fp16. Instead, the accumulation dtype should be in fp32. i.e.

dtype = fp16 -> acc_type = fp32
dtype = fp32 -> acc_type = fp64
dtype = fp64 -> acc_type = fp64

We should do it for the norm op.

Reference impl for softmax: #14098

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