fix bf16 constant accuracy (#105827)
This PR aims to sort out the data type for `constant`.
The constant should be promoted to float https://github.com/pytorch/pytorch/pull/105440. So there are serval changes to do:
- Data type propagation should propagate constant node to `float` dtype if original dtype is `bfloat16`
- We do not need to insert `to_dtype` after the `constant` node, directly init an `fp32` constant is faster.
```
vectorized<bfloat16> tmp(value);
vectorized <float> tmp1 = cvt_bf16_fp32(tmp);
->
vectorized<float> tmp(value);
```
- move `constant` out of the list for `all operations can support bf16 without converting to fp32`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105827
Approved by: https://github.com/jgong5, https://github.com/jansel
