change the epsilon for fp32/fp16 to uint8 to be the same (#17062)

change the epsilon for fp32/fp16 to uint8 to be the same (#17062)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17062

from jiyan's training jobs it seems like we found a quantization bug

fp32
fp32->rowwise int8 is fine
fp16 is fine
fp16->rowwise int8 is not fine

we are preconverting everything to fp32 and using the existing code, so there is no need to change the epsilon in the case of fp16 since at the time of converting, everything is a float

Reviewed By: jspark1105

Differential Revision: D14063271

fbshipit-source-id: 747297d64ed8c6fdf4be5bb10ac584e1d21a85e6