[quant][pt2e] Relax `model_is_exported` input (#120720)
Summary: This commit relaxes the `model_is_exported` API to
additionally work for `torch.nn.Module`s in addition to just
`torch.fx.GraphModule`s, simplifying downstream uses.
Test Plan:
python test/test_quantization.py TestQuantizePT2E.test_model_is_exported
Differential Revision: [D54263935](https://our.internmc.facebook.com/intern/diff/D54263935)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120720
Approved by: https://github.com/tugsbayasgalan
