Rename _internal/exporter to _exporter_legacy (#132429)
Summary:
The next PR will be creating an `exporter` directory to house logic from `torch-onnx`
X-link: https://github.com/pytorch/pytorch/pull/132429
Approved by: https://github.com/titaiwangms
Reviewed By: PaliC
Differential Revision: D60781226
fbshipit-source-id: c73e961c3cf7c752ab3fa7fc7912013355244339
