add mixed data type support for LayerNorm (#81851)
1. If user uses amp to run bfloat16 models, `torch.autocast` will
keep module paramters in acc dtype which will leave `gamma` and`beta`
in float while input/output will be in bfloat16.
2. If user explicitly cast the model to bfloat16 such as:
```
x = torch.randn(n, t, c).bfloat16()
ln = nn.LayerNorm(c).bfloat16()
y = ln(x)
```
The input/output and gamma/beta will all be in bfloat16.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81851
Approved by: https://github.com/ezyang
