Take gradient accumulation into account when defining samplers
# What does this PR do?

This PR takes the gradient accumulation steps into account when defining samplers that use the batch size (like the `LengthGroupedSampler`) so that training with large batch size or training with smaller batch size and gradient accumulation (e.g. batch size 64 or batch size 8 and gradient accumulation steps of 8) yield the same results.

Fixes #14638 