observers: make eps a buffer (#43149)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43149
This value doesn't change, making it a buffer to only pay
the cost of creating a tensor once.
Test Plan: Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D23170428
fbshipit-source-id: 6b963951a573efcc5b5a57649c814590b448dd72
