[quant][pt2e][be] Rename eval_utils -> export_utils (#119725)
It's not really eval_utils anymore, since we added some training
related utils. Instead it should be util functions that are
related to general export use cases.
Differential Revision: [D53711494](https://our.internmc.facebook.com/intern/diff/D53711494)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119725
Approved by: https://github.com/tugsbayasgalan
