Do not use "buffer" in reentrant autograd err msg (#38625)
Summary:
`buffer` is also used to refer to `nn.Module`'s buffer. Wording is changed to reduce confusion between the two.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38625
Differential Revision: D21629396
Pulled By: albanD
fbshipit-source-id: acb5ef598739efabae7b388e1a4806c9caf0f589
