Fix minor grammatical mistakes in torch/nn/modules/loss.py (#17892)

Fix minor grammatical mistakes in torch/nn/modules/loss.py (#17892)

Summary:
Fixes some minor grammatical mistakes in the doc of `loss.py`.

I think in the doc:
>  Note that for some losses, there multiple elements per sample.

the "are" is lost between "there" and "multiple".

This mistake takes place in all the descriptions of parameter `size_average` and there are 17 of them.
It's minor but perfects the doc I think. 😁
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17892

Differential Revision: D14418177

Pulled By: ezyang

fbshipit-source-id: 412759f2f9b215819463bf8452ab0e0513218cd6