Fix batch size in evaluation loop
# What does this PR do?

The batch size used in the evaluation loop is wrong: it's using the per device batch size, which is different from the actual batch size when using DataParallel with more than one GPU. As a result, the `test_evaluate` test is failing for 2 GPUs (see #16716).

This PR fixes that.
_The documentation is not available anymore as the PR was closed or merged._